Aerospace Science and Technology Dictionary D Page

Aerospace Science
&
Technology Dictionary

D

D region: A daytime layer of the Earth's ionosphere about 50 to 100 km in altitude above sea level.
D region: The lowest layer of the Earth's ionosphere. It is between about 50 and 95 kilometers above Earth's surface. This is the layer which reflects radio waves. Also called the D Layer.
D-display: In radar, a C-display in which the blips extend vertically to give a rough estimate of distance. Also called D-indicator, D-scan, D-scope.
D-indicator: = D-display.
D-layer: See ionosphere.
D-region: See ionosphere.
D-scan: = D-display.
D-scope: = D-display.
dabble: See double-dabble.
dacite: Dacite is a light gray rock with 63 to 68 percent silica (SiO2). It is one of the most common rock types associated with enormous Plinian eruptions. Common minerals include plagioclase feldspar, pyroxene, amphibole and iron oxide. Dacite magmas generally erupt at temperatures between 800 and 1000°C.
Dalton: = atomic mass unit.
Dalton law: The empirical generalization that for many so-called perfect gases, a mixture of these gases will have a pressure equal to the sum of the partial pressures that each of the gases would have as sole component with the same volume and temperature, provided there is no chemical interaction.
Dalton law of partial pressure: = Dalton law.
damage assessment: Estimate of injury or loss to components, subsystems, or entire systems, as well as the cost of repairs or replacement to restore serviceability.
damp: To suppress oscillations or disturbances.
damped natural frequency: The frequency of free vibration of a damped linear system.
The oscillation of a damped system may be considered periodic in the limited sense that the time interval between zero crossings in the same direction is constant if the system is linear, even though successive amplitudes decrease progressively. The frequency of the oscillation is the reciprocal of this time interval. The damped natural frequency decreases as the damping increases, and approaches zero as the damping approaches critical damping.
damped wave: Any wave whose amplitude decreases with time or whose total energy decreases by transfer to other portions of the wave spectrum.
damping: The suppression of oscillations or disturbances; the dissipation of energy with time. See viscous damping.
damping factor: The ratio of the amplitude of any one of a series of damped oscillations to that of the following one at the same phase.
damping ratio: The ratio of actual damping to critical damping.
It may be expressed as the ratio of output under static conditions to twice the output at the lowest frequency where a 90° phase shift is observed.
dark adaptation: The process by which the iris and retina of the eye adjust to allow maximum vision in dim illumination, following exposure of the eye to a relatively brighter illumination.
dark blips: See dark trace tube.
dark terrain: A type of surface on Ganymede and Callisto which is darker than other regions on these moons (see also bright terrain). On Ganymede dark terrain is often associated with furrows.
dark trace tube: A cathode-ray tube, on which the face is bright, and signals are displayed as dark traces or dark blips.
dart configuration: A configuration of an aerodynamic vehicle in which the control surfaces are at the tail of the vehicle. Contrast canard.
DAST program: A NASA program which uses the Firebee 2 target drone aircraft as a test bed for getting flight data on research wings. The drone is launched from the wing of a B52 and recovered by parachute. The program's purpose is the study of flight loads and load control. Used for drones for aerodynamic and struct test.
data base management systems: Software products that control data structures containing interrelated data stored so as to optimize accessibility and control, minimize redundancy, and offer multiple views of the data to various applications programs.
data compression: Any technique used to reduce the amount of storage required to store data.
data integration: Taking data from multiple sources and merging the data into a single data file.
data link: Any communications channel or circuit used to transmit data from a sensor to a computer, a readout device, or a storage device.
data point: A unit of fundamental information obtained through the processing of raw data.
data processing: Application of procedures, mechanical, electrical, computational, or other, whereby data are changed from one form into another.
data processing equipment: Machines for handling information in a sequence of reasonable operations. Used for data processors.
data processor: A machine for handling information in a sequence of reasonable operations.
data reduction: Transformation of observed values into useful, ordered, or simplified information.
data simulation: The use of statistical or physical models to produce synthetic data for testing purposes.
data smoothing: The mathematical process of fitting a smooth curve to dispersed data points.
data structures: The organization of computer memory used to represent information in a computer program or database.
data transfer (computers): The technique used by the hardware manufacturer to transmit data from computer to storage device or from storage device to computer, usually under specialized program control.
data-acquisition station: A ground station at which various functions to control satellite operations and to obtain data from the satellite are performed.
datum: Any numerical or geometrical quantity or set of such quantities which can serve as a reference or a base for measurement of other quantities.
For a group of statistical references, the plural form is data; as geographic data for a list of latitudes and longitudes. Where the concept is geometrical the plural form is datums; as in two geodetic datums have been used.
datum (elevation): The arbitrary "zero plane" from which all stage measurements are taken from. Usually set below the natural bottom of the channel so all stage height readings will be greater than zero.
datum line: Any line which can serve as a reference or base for the measurement of other quantities.
datum plane: A plane from which angular or linear measurements are reckoned. Also called reference plane.
datum point: Any point which can serve as a reference or base for the measurement of other quantities.
daughter, daughter element: = decay product.
dawsonite: A mineral consisting of aluminum sodium carbonate.
day: 1. The duration of one rotation of the earth, or another celestial body, on its axis.
A day is measured by successive transits of a reference point on the celestial sphere over the meridian, and each type takes its name from the reference used. Thus, for a solar day the reference is the sun; a mean solar day if the mean sun; and an apparent solar day if the apparent sun. For a lunar day the reference is the moon; for sidereal day the vernal equinox; for a constituent day an astre fictif or fictitious star. The expression lunar day refers also to the duration of one rotation of the moon with respect to the sun. A Julian day is the consecutive number of each day, beginning with January 1, 4713 BC.
2. A period of 24 hours beginning at a specified time, as the civil day beginning at midnight, or the astronomical day beginning at noon.
daylight saving time: See time.
daytime visual range: = visual range.
DCRsi: The Digital Cassette Recording system incremental (DCRsi) rack mount and modular ruggedized systems are one-inch, transverse scan, rotary digital recorders capable of recording and reproducing at any user data rate from 0 to 13.4 Mbytes/seconds (0-107 Mbits/seconds). DCRsi is a user friendly mass storage data peripheral with a total storage capacity of 48 gigabytes.
DDR--Data Descriptor Record: A DDR is a file containing image information which may include: (1) number of lines, number of samples, number of bands, data type, and the system on which the data was created; (2) corner coordinates of the image and related projection information; (3) the minimum and maximum values for each band of an image; (4) information describing how and when each band of the image was acquired; and (5) miscellaneous information (e.g., the last date and time modifications were made to an image).
De Broglie wavelength: For a particle of mass m and velocity v , the De Broglie wavelength, = h/mv, where h is Planck constant.
de Broglie wavelengths: According to quantum mechanics all particles also have wave characteristics, where the wavelength of a particle is inversely proportional to its momentum and the constant of proportionality is the Planck constant.
De Laval nozzle: A device for efficiently converting the energy of a hot gas to kinetic energy of motion, originally used in some steam turbines and now used in practically all rockets. By constricting the outflow of the gas until it reaches the velocity of sound and then letting it expand again, an extremely fast jet is produced.
de Laval nozzle: [After Dr. Carl Gustaf Patrik de Laval (1845-1913), Swedish engineer.] A converging-diverging nozzle used in certain rockets. Also called Laval nozzle.
dead band: An arrangement incorporated in a guidance system which prevents an error from being corrected until that error exceeds a specified magnitude.
dead reckoning: (abbr DR)
In navigation, determination of position by advancing a previous known position for courses and distances.
dead spot: In a control system, a region centered about the neutral control position where small movements of the actuator do not produce any response in the system.
dead time: In a radiation counter, the time interval, after the start of a count, during which the counter is insensitive to further ionizing events.
debris avalanche: Debris avalanches are moving masses of rock, soil and snow that occur when the flank of a mountain or volcano collapses and slides downslope. As the moving debris rushes down a volcano and into river valleys, it incorporates water, snow, trees, bridges, buildings, and anything else in the way. Debris avalanches may travel several kilometers before coming to rest, or they may transform into more water-rich lahars, which travel many tens of kilometers downstream.
debris cloud: A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado.
debug: 1. To isolate and remove malfunctions from a device, or mistakes from a routine or program.
2. Specifically, in electronic manufacturing, to operate equipment under specified environmental and test conditions in order to eliminate early failures and to stabilize equipment prior to actual use. Also called burn-in.
Debye length: A theoretical length which describes the maximum separation at which a given electron will be influenced by the electric field of a given positive ion. Sometimes referred to as the Debye shielding distance or plasma length.
It is well known that charged particles interact through their own electric fields. In addition, Debye has shown that the attractive force between an electron and ion which would otherwise exist for very large separations is indeed cut off for a critical separation due to the presence of other positive and negative charges in between. This critical separation or Debye length decreases for increased plasma density.
Debye shielding: If a positive (or negative) charge is inserted into a plasma, it will change the local charge distribution by attracting (repelling) electrons. The net result is an additional negative (positive) charge density which cancels the effect of the initial charge at distances large compared to the Debye length.
Debye shielding distance: = Debye length.
DEC: Declination, the measure of a celestial body's apparent height above or below the celestial equator.
deca: = deka.
decade: 1. The interval between any two quantities having the ratio of 10:1.
2. A group of series of 10.
decade counter: A counter that counts to 10 in one column of decimal notation; a scale of 10 counter.
decametric wave: See frequency band.
decay: Decrease of a radioactive substance because of nuclear emission of alpha or beta particles, positrons, or gamma rays. See radioactivity. In beta decay, for example, the emission of a -particle, i.e., an electron, causes radioactive change into a daughter element of the same atomic weight as the parent element but of atomic number higher by 1.
decay constant: 1. = attenuation constant.
2. (symbol ) A constant relating the instant rate of radioactive decay of a radioactive species to the number of atoms N present at a given time t. Thus,

-(N / t) = N
_{If N_o is the number of atoms present at time

zero then}
Decay modes: Different pathways for decay of radioactive nuclei. The decay modes for a given unstable state can include beta emission (negative = electron, positive = positron), electron capture, alpha emission, fission, and gamma emission.
decay product: A nuclide resulting from the radioactive disintegration of a radionuclide, being formed either directly or as the result of successive transformations in a radioactive series. Also called daughter, daughter element.
A decay product may be either radioactive or stable.
decay time: 1. In computer operations, the time required for a pulse to fall to one-tenth of its peak value. See rise time.
2. In charge-storage tubes, the time interval during which the magnitude of the stored charge decreases to a stated fraction of its initial value.
The fraction is usually 1/e where e is the base of natural logarithms.
3. Approximately the lifetime of an orbiting object in a nonstable orbit.
Decay time is usually applied only to objects with short orbit lifetimes caused by atmospheric drag.
decayed object: An object once, but no longer, in orbit.
Decca: A long-range, ambiguous, two-dimensional navigation system using continuous-wave transmission to provide hyperbolic lines of position through the radio frequency phase comparison techniques from four transmitters.
Frequency band, 68 to 150 kilocycles.
Decca navigation: A long range, ambiguous, two dimensional navigation system using continuous wave transmission to provide hyperbolic lines of position through the radio frequency phase comparison techniques from four transmitters.
decelerate: To cause to move slower; to decrease speed.
deceleration: The act or process of moving, or of causing to move, with decreasing speed. Sometimes called negative acceleration.
deceleration parachute: A parachute attached to a craft and deployed to slow the craft, especially during landing. Also called a brake parachute , drogue parachute, parabrake.
December solstice: = winter solstice.
deci: (abbr d)
A prefix meaning multiplied by 10^-1; one-tenth.
decibel: (abbr db)
1. A dimensionless measure of the ratio of two powers, equal to 10 times the logarithm to the base 10 of the ratio of two powers P₁/P₂.
2. One-tenth of a bel.
The power P₂ may be some reference power; in electricity, the reference power is sometimes taken as 1 milliwatt.
decibel per second: A unit used to measure the rate of decay of a sound.
decimal coefficient of absorption: See absorption coefficient.
decimal digit: 1. One of the digits used in decimal notation, i.e., 1,2,3,4,5,6,7,8,9, or 0.
2. One of 10 possible conditions.
decimal notation: A mathematical system in which each digit is the coefficient of some power of 10.
decimal point: The base point in decimal notation.
decimal-to-binary conversion: The mathematical process of converting a quantity from decimal notation to the equivalent binary notation. For example: 1 = 1; 7 = 111; 23 = 10111, etc. See binary notation.
decimetric wave: See frequency band.
decimillimetric waves: See frequency bands.
decision element: In computer operations, any device which as the result of the input of data issues one of two or more possible instructions.
declination: 1. (symbol ) Angular distance north or south of the celestial equator; the arc of an hour circle between the celestial equator and a point on the celestial sphere, measured northward or southward from the celestial equator through 90 degrees, and labeled N or S to indicate the direction of measurement.
2. (symbol D) Magnetic declination. See equatorial system.
declination: Declination is the angular distance on the celestial sphere north or south of the celestial equator. It is measured along the hour circle passing through the celestial object.
decoder: 1. A device for translating electrical signals into predetermined functions.
2. In computer operation, a network or device in which one of two or more possible outputs results from a prescribed combination of inputs. Also called many-to-few matrix.
decoders: Devices for translating electrical signals into predetermined functions. In computer operations, networks or devices in which one of two or more possible outputs results from a prescribed combination of inputs.
decommissioning: Disposal or deactivation of equipment or sites whose usefulness has diminished to a point where it is no longer required for its original purpose.
decommutator: Equipment for separation, demodulation, or demultiplexing commutated signals. See commutator.
decompression: The reduction of atmospheric pressure; particularly, various techniques for preventing caisson disease.
decompression sickness: A disorder experienced by deep sea divers and aviators caused by reduced atmospheric pressure and evolved gas bubbles in the body, marked by pain in the extremities, pain in the chest (chokes), occasionally leading to serve central nervous symptoms and neurocirculatory collapse. See bends, dysbarism.
decontamination: The act of removing chemical, biological, or radiological contamination from, or neutralizing it on, a person, item, or area.
deconvolution: An image processing technique that removes features in an image that are caused by the telescope itself rather than from actual light coming from the sky.
decoupled: Of circuits or devices, interconnected through any means which passes only the static characteristics of a signal.
decrement: A decrease in the value of a variable. See increment.
decrement gage: A gage in which pressure is measured by the rate of decay in amplitude of the oscillations of an element suspended in the gas and set into motion by external controls. Also known as decrement viscosity gage or viscosity manometer.
Various types of decrement gage are distinguished according to the design of the oscillating element.
decrement viscosity gage: = decrement gage.
Deep Space 1 Mission: First of several technology demonstration missions supporting the NASA New Millennium Program. Advanced technologies include an ion propulsion system, solar concentrator arrays, autonomous navigation and control systems, an integrated camera and imaging spectrometer, and several telecommunications and microelectronics devices. The mission plan includes a flyby of Asteroid 1992 KD.
Deep Space Network: A communications network managed by the Jet Propulsion Laboratory for command and control of all planetary flights.
deep well injection (wastes): Storage of liquid wastes, particularly chlorohydrocarbons, by injection into subsurface geologic strata for long term isolation from the environment.
definition: The clarity, fidelity, sharpness, resolution and brilliancy of an image, as a photographic image.
deflagration: A sudden or rapid burning, as opposed to a detonation or explosion.
deflecting force: = coriolis force.
deflection of the vertical: The angular difference, at any place, between the direction of plumb line (the vertical) and the perpendicular (the normal) to the reference spheroid. This difference seldom exceeds 30 seconds of arc. Also called station error.
When measured at the earth's surface the deflection of the vertical is equal to the angle between the geoid and the reference spheroid.
deflection-modulated indicator: = amplitude-modulated indicator.
deflector: A plate, baffle, or the like that diverts something in its movement or flow; as: (a) a plate that projects into the airstream on the underside of an airfoil to divert the airflow, as into a slot-sometimes distinguished from a spoiler; (b) a conelike device placed or fastened beneath a rocket launched from the vertical position, to deflect the exhaust gases to the sides; (c) any of several different devices used on jet engines to reverse or divert the exhaust gases; (d) a baffle or the like to deflect and mingle fluids prior to combustion, as in certain jet engines.
deformation: A change in the shape or size of a solid body.
degas: To remove gas from a material, usually by heating under high vacuum. Compare get.
degassing: The deliberate removal of gas from a material, usually by application of heat under high vacuum.
degauss: Slang for demagnetize.
degenerate matter: A state of matter found in white dwarf stars and other ultrahigh-density objects in which the electrons follow Fermi-Dirac statistics, i.e., the matter reaches a density high enough so that the pressure increases more and more rapidly to the point where it becomes independent of the temperature and is a function of the density only, thereby departing from the classical laws of physics.
degradation: Gradual deterioration in performance.
degree of condensation (DC): DC is an indicator of how much the surface brightness of the coma increases toward the center of the coma. As the DC increases, the coma size usually decreses and becomes more sharply defined.
degree of freedom: 1. A mode of motion, either angular or linear, with respect to a coordinate system, independent of any other mode.
A body in motion has six possible degrees of freedom, three linear and three angular.
2. Specifically, of a gyro the number of orthogonal axes about which the spin axis is free to rotate.
3. In an unconstrained dynamic or other system, the number of independent variables required to specify completely the state of the system at a given moment.
If the system has constraints, i.e., kinematic or geometric relations between the variables, each such relation reduces by one the number of degrees of freedom of the system. In a continuous medium with given boundary conditions, the number of degrees of freedom is the number of normal modes of oscillation.
4. Of a mechanical system, the minimum number of independent generalized coordinates required to define completely the positions of all parts of the system at any instant of time.
In general, the number of degrees of freedom equals the number of independent generalized displacements that are possible.
degrees of freedom: A mode of motion, either angular or linear, with respect to a coordinate system, independent of any other mode. A body in motion has six possible degrees of freedom, three linear and three angular.
dehumidification: The reduction, by any process, of the quantity of water vapor within a given space.
Deimos: A satellite of Mars orbiting at a mean distance of 23,500 kilometers.
deionization: The removal of ions from a solution by ion exchange.
deka: (abbr da)
A prefix meaning multiplied by 10. Sometimes spelled deca.
Del, Dlph: International Astronomical Union abbreviations for Delphinus. See constellation.
del-operator: (symbol )
The operator used in vector calculus and defined in Cartesian coordinates as
delay: The time (or equivalent distance) displacement of some characteristic of a wave relative to the same characteristic of a reference wave; that is, the difference in phase between the two waves. Compare lag.
In one-way radio propagation, for instance, the phase delay of the reflected wave over the direct wave is a measure of the extra distance traveled by the reflected wave in reaching the same receiver.
delay element: A device for causing time delay of a signal. See delay line.
delay line: In electronic computers, any device for producing a time delay of a signal.
delay lines (computer storage): In electronic computers, devices for producing a time delay of a signal.
delay-line storage: A storage or memory device consisting of a delay line and means for regenerating and reinserting information into the delay line.
delayed neutron activation analysis: This non-destructive analytical technique is used to determine concentrations of specific chemical elements. The procedure is based on artificially induced neutron capture and the radioactive decay constants of unstable radionuclides that are produced.
delayed neutrons: Neutrons emitted by excited nuclei in a radioactive process, so called because they are emitted an appreciable time after the fission. Compare prompt neutrons.
delayed plan position indicator: See plan position indicator.
delayer: A substance mixed in with solid rocket propellants to decrease the rate of combustion.
Dellinger effect: = fadeout.
Delphinus: (abbr Del, Dlph)
See constellation.
delta ray: 1. An electron ejected by recoil when a rapidly moving alpha particle or other charged particle passes through matter.
2. By extension any secondary ionizing particle ejected by recoil when a primary particle passes through matter.
delta wing: A triangularly shaped wing of an aircraft.
deluge collection pond: A facility at a launch site into which used to cool the flame deflector is flushed as the rocket begins its ascent. Also called a skimmer basin.
DEM--Digital Elevation Models: The U.S. Geological Survey produces five primary types of digital elevation model data. They are 1) 7.5-minute DEM, which provides coverage of the contiguous United States, Hawaii, and Puerto Rico. in 7.5- x 7.5-minute blocks. 2) 1-degree DEM, which provides coverage of the United States in 1- x 1-degree blocks. 3) 30-minute DEM (2- x 2-arc-second data spacing), consisting of four 15- x 15-minute DEM blocks that cover the contiguous United States and Hawaii. And finally 4 and 5) 15-minute Alaska DEM and 7.5-minute Alaska DEM, both of which provide coverage of Alaska, but with different spacing.
demagnetization: The reduction of residual magnetism to an acceptable level.
demand assignment multiple access: A technique of assigning communication resources on an "as needed basis" such as in satellite communications. Used for DAMA.
demand oxygen system: = demand system.
demand system: An oxygen system in which oxygen flows to the user during inspiration only.
demodulation: The process of recovering a signal from a modulated (varied frequency) carrier wave.
demodulation: The process of recovering the modulating wave from a modulated carrier.
demodulator: An electronic device which operates on an input of a modulated carrier to recover the modulating wave as an output.
demodulators: Electronic devices which operate on an input of a modulated carrier to recover the modulating wave as an output.
demography: Statistical study of human populations, especially with reference to size, density, distribution, and vital data.
demultiplexing: Separation of two or more signals that were previously combined by a compatible multiplexer and transmitted over a single channel.
dendrochronology: The use of annual growth rings in plant tissue to determine the age of the plant or tree. Used for tree ring dating.
denitrogenation: The removal of nitrogen dissolved in the blood and body tissues, usually by breathing of pure oxygen for an extended period of time in order to prevent aeroembolism at high altitudes.
densimeters: Instruments for measuring the density or specific gravity of liquids, gases, or solids.
densitometer: An instrument for the measurement of optical density (photographic transmission, photographic reflection, visual transmission, etc.) of a material, generally of a photographic image.
density: A property of matter measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0; iron is 7.9; lead is 11.3.
density (mass/volume): The mass per unit volume of a material at a specified temperature.
density function: The number of particles per unit volume. See distribution function.
density specific impulse: The product of the specific impulse of a propellant combination and the average specific gravity of the propellants.
deoxyribonucleic acid: The molecule that encodes genetic informatiion - a double-stranded moleculeeld together by weak bonds betweeen base pairs of nucleotides. Used for DNA.
departure: = deviation, sense 1.
dependent variable: Any variable considered as a function of other variables, the latter being called independent. Compare parameter.
Whether a given quantity is best treated as a dependent or independent variable depends upon the particular problem.
dependent variables: Variables considered as a function of other variables, the latter being called independent.
depletion layer: In a semiconductor, a region in which the mobile carrier charge density is insufficient to neutralize the net fixed charge density of donors and acceptors. Also called barrier.
deploy: Of a parachute, to release so as to let it fill out or to unfold and fill out.
depolarization: A decrease in the polarization of an electrode at a specified current density. Used for depolarizers.
depressed pole: The celestial pole below the horizon, of opposite name to the latitude.
The celestial pole above the horizon is called the elevated pole.
depression angle: = angle of depression.
depth perception: The ability to estimate depth or distance between points in the field of vision.
derivative data: Data which have been derived from other data by mathematical techniques.
descending node: The point at which an orbit crosses the ecliptic plane going south.
descending node: That point at which a planet, planetoid, or comet crosses to the south side of the ecliptic; that point at which a satellite crosses to the south side of the equatorial plane of its primary. Also called southbound node. The opposite is ascending node or northbound node.
desertification: The formation of a desert or the gradual expansion of a desertline into previously usable land, due to man-made or natural causes.
desiccants: Chemicals used to absorb moisture.
design gross weight: The gross weight at take-off that an aircraft, rocket, etc., is expected to have, used in design calculations.
design to cost: A process whereby cost factors are determined and calculated for the life cycle of a product as an integral part of its design.
desorption: The process of removing sorbed gas.
destruct: The deliberate action of destroying a rocket vehicle after it has been launched, but before it has completed its course.
Destructs are executed when the rocket gets off its plotted course or functions in a way so as to become a hazard.
destruct line: On a rocket test range, a boundary line on each side of the downrange course beyond which a rocket cannot fly without being destroyed under destruct procedures, or a line beyond which the impact point cannot pass. See impact line, command destruct.
desynchronization (biology): The loss of synchronization between two or more rhythms so that they show independent periods.
detached shock: = detached shock wave.
detached shock wave: A shock wave not in contact with the body which originates it. See bow wave. Also called detached shock.
detachment: A particular state of isolation in which man is separated or detached from his accustomed behavioral environment by inordinate physical and psychological distances. This condition may compromise his performance.
detection: See recognition, note.
detector: 1. = sensor, sense 1.
2. An instrument employing a sensor, sense 1, to detect the pressure of something in the surrounding environment.
detectors: Sensors or instruments employing a sensor.
detonation: A rapid chemical reaction which propagates at a supersonic velocity.
detonation wave: A shock wave in a combustible mixture, which originates as a combustion wave.
detonation waves: Shock waves that accompany detonation and have a shock front followed by a region of decreasing pressure in which the reaction occurs.
deuterium: (symbol D, d)
A heavy isotope of hydrogen having one proton and one neutron in the nucleus.
The symbol D is often used to designate deuterium in compounds, as HDO for molecules of that composition. Official chemical nomenclature uses the designation d with a number which designates the carbon atom to which deuterium is bound; e.g. 2-d propane designates CH₃CHDCH₃.
deuterium fluorides: Fluorides of deuterium, a heavy isotope of hydrogen. Used for DF.
deuteron: The nucleus of a deuterium atom.
deuteron (differs from Thesaurus): Deuteron: A deuterium ion; nucleus consisting of a proton and a neutron.
deuterons: The nuclei of deuterium atoms.
deviation: The variation from a specified dimension or design requirement, usually defining upper and lower limits.
deviation: 1. In statistics, the difference between two numbers. Also called departure.
It is commonly applied to the difference of a variable from its mean, or to the difference of an observed value from a theoretical value.
2. = magnetic deviation.
3. In radio transmission, the apparent variation of frequency above and below the unmodulated or center frequency.
dew point: The temperature to which a given parcel of air must be cooled at constant pressure and constant water-vapor content in order for saturation to occur; the temperature at which the saturation vapor pressure of the parcel is equal to the actual vapor pressure of the contained water vapor. Any further cooling usually results in the formation of dew or frost. Also called dewpoint temperature.
When this temperature is below 0° C, it is sometimes called the frost point.
dew-point temperature: = dew point.
dewar: Cryogenic storage container in which two concentric vessels are separated by an insulating vacuum which prevents conductive and convective heat losses. The vessels themselves are often silvered to reduce radiative heat losses.
dewatering: Removal of water by draining, pumping, or other means.
DF (abbr): = direction finder.
See radio direction finder.
DF lasers: Gas lasers in which the active material is deuterium fluoride. Used for deuterium fluoride lasers.
DGL optional format: The USGS digital line graph (DLG-3) optional format, which was designed for data interchange, allows for the creation of a vector polygon data structure. The topological linkages are explicitly encoded for node, area, and line elements. The files are composed of 8-bit ASCII characters organized into fixed logical records of 80 bytes. Bytes 1 - 72 contain the data, bytes 73 - 77 are blank, and bytes 78 - 80 contain a record sequence number. The detailed description of the DLG-3 optional format is described in Digital Line Graphs from 1:2,000,000-Scale Maps, Data Users Guide 3 (1990).
DHC 2 aircraft: De Havilland Canada STOL utility aircraft. Used for DHC Beaver aircraft.
diabatic process: A process in a thermodynamic system in which there is a transfer of heat across the boundaries of the system.
Diabatic process is preferred to nonadiabatic process.
diameters: Lengths of the longest straight lines through the centers of the largest cross sections.
diamonds: (A) An isometric mineral, representing a naturally occurring crystalline form of carbon dimorphous with graphiteand being the hardest natural substance known. (B) Artificially produced crystallized carbon similar to the native form. (C) A crystalline mineral that resembles diamonds in brilliance.
diamonds: The pattern of shock waves often visible in a rocket exhaust which resembles a series of diamond shapes placed end to end.
diaphragm (anatomy): Musculomembranous partition separating the abdominal and thoracic cavities.
diaphragm manometer: A displacement manometer employing a flexible diaphragm as the movable partition.
diastolic blood pressure: The pressure exerted by the blood during periods between cardiac contraction.
dichotomy: In astronomy, a configuration of three bodies so that they form a right triangle; specifically, such a configuration in the solar system with the sun at the apex of the 90° angle.
didymium: A mixture of rare earth elements that is freed from cerium. It was once regarded as an element but contains chiefly neodymium and praseodymium and is usually associated with lanthanum. It is used in coloring glass for optical filters.
dielectric: A substance that contains few or no free charges and which can support electrostatic stresses.
In an electromagnetic field, the centers of the nonpolar molecules of a dielectric are displaced, and the polar molecules become oriented close to the field. The net effect is the appearance of charges at the boundaries of the dielectric. The frictional work done in orientation absorbs energy from the field which appears as heat. When the field is removed the orientation is lost by thermal agitation and so the energy is not regained. If free-charge carriers are present they too can absorb energy.
A good dielectric is one in which the absorption is a minimum. A vacuum is the only perfect dielectric. The quality of an imperfect dielectric is its dielectric strength; and the accumulation of charges within an imperfect dielectric is termed dielectric absorption.
dielectric absorption: See dielectric.
dielectric constant: (symbol )
For a given substance, the ratio of the capacity of a condenser with that substance as dielectric to the capacity of the same condenser with a vacuum for dielectric. It is a measure, therefore, of the amount of electrical charge a given substance can withstand at a given electric-field strength; it should not be confused with dielectric strength.
The dielectric constant is a function of temperature and frequency and is written as a complex quantity where ' is the part that determines the displacement current and '' the dielectric absorption (see dielectric). For a nonabsorbing, nonmagnetic material ' is equal to the square of the index of refraction and the relation holds only at the particular frequency where these conditions apply.
dielectric gradient: The spatial variation of the dielectric constant in a substance or medium.
This term is used frequently in reference to the propagation of radio energy. The magnitude of these gradients and the distance over which they occur, relative to the wavelength of the incident radiation, determine the extent to which targets will reflect radar energy. Sufficiently intense dielectric gradients (or index of refraction gradients) are believed to be the cause of certain echoes known as angels.
dielectric strength: A measure of the resistance of a dielectric to electrical breakdown under the influence of strong electric fields; usually expressed in volts per centimeter. Sometimes called breakdown potential.
dielectrics: Substances that contain few or no free charges and which can support electrostatic stresses. Used for dielectric materials.
difference of latitude: The shorter arc of any meridian between the parallels of two places, expressed in angular measure.
difference of longitude: The smaller angle at the pole or the shorter arc of a parallel between the meridians of two places, expressed in angular measure.
differential analyzer: An analog computer designed and used primarily for solving differential equations.
differential correction: In celestial mechanics, a method for finding from the observed residuals minus the computed residuals (O - C) small corrections which, when applied to the orbital elements or constants, will reduce the deviations from the observed motion to a minimum.
differential manometer: A manometer which indicates the pressure difference across two ports.
differential pressure: The pressure difference between two systems or volumes.
differential pulse code modulation: An efficient signal encoding method of reducing the transmission rate of digital signals. The basic principle of DPCM is to quantize code and transmit the difference between the actual sample and prediction value. Used for DPCM (modulation).
differential rotation: The change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees per day) than do high latitudes (approx. 12 degrees per day).
differential thermal analysis: The technique of detecting endothermic and exothermic phase changes and other processes within a heated material by the corresponding temperature changes.
differential transducer: A device which is capable of measuring simultaneously two separate stimulus sources and which provides an output proportional to the difference between the stimuli. See transducer.
differentiator: 1. In computer operations, a device whose output is proportional to the derivative of an input signal.
2. In electronics, a transducer whose output waveform is the time derivative of its input waveform.
differentiators: In computer operations, devices whose output is proportional to the derivative of an input signal. In electronics, a transducer whose output waveform is the time derivative of its input waveform.
diffracted wave: A wave whose front has been changed in direction by an obstacle or other nonhomogeneity in a medium, other than by reflection or refraction.
diffraction: The process by which the direction of radiation is changed so that it spreads into the geometric shadow region of an opaque or refractive object that lies in a radiation field.
Diffraction is an optical edge effect.
Reference to Huygens' principle is a common means of explaining diffraction. Analysis of the interference between individual Huygens wavelets which originate in the vicinity of the edge of an irradiated body reveals that detectable amounts of radiant energy must invade the nominal shadow zone of the object, and there, by interference, set up characteristic energy distributions known as diffraction patterns. The amount of diffractive bending experienced by a ray is a function of wavelength; thus dispersion occurs, although dispersion is in the opposite sense to that produced by refraction.
diffraction grating: A flat optical surface, transparent or reflecting, ruled with many parallel grooves at precisely spaced distances. The active parts are not the grooves but the flat sections left between them, which act like a large number of precisely spaced slits. The light waves passing those slits resonate with each other in a way which depends on wavelength, causing different wavelengths to be steered in different directions. The overall effect on light containing different wavelengths is like that of a glass prism: the intensity of the light deflected is much smaller than with a prism, but the ability to separate close colors is much better.
diffraction propagation: Wave propagation around objects, or over the horizon, by diffraction.
Diffraction is due to the fact that from every point in a wave front a spherical front is generated which falls off in intensity away from the forward direction. A continuous series of such actions carries radiation around objects, or around the curvature of the earth, but with rapidly diminishing intensity.
diffraction radiation: Electromagnetic radiation excited by an electron flux passing near a diffractive, periodic structure, such as a wiggler magnet in a free electron laser.
diffractive optics: Optical elements that add up scattered light from a multiple of disturbances in amplitude or phase to generate a transformed wavefront.
diffuse aurora: A spread-out glow often covering much of the auroral oval. It is not seen by the eye but can be observed quite well by satellite cameras.
diffuse radiation: Radiant energy propagating in many different directions through a given small volume of space; to be contrasted with parallel radiation.
The ideal form of diffuse radiation is isotropic radiation. Careful distinction should be made between this concept and that of a perfectly diffuse radiator.
diffuse reflection: Any reflection process in which the reflected radiation is sent out in many directions usually bearing no simple relationship to the angle of incidence; the opposite of specular reflection. See diffuse reflector, perfectly diffuse reflector.
A term frequently applied to the process by which solar radiation is scattered by dust and other suspensoids in the atmosphere. See diffuse sky radiation.
diffuse reflector: Any surface which reflects incident rays in a mutliplicity of directions, either because of irregularities in the surface or because the material is optically inhomogeneous, as a paint, although optically smooth; the opposite of a specular reflector. See perfectly diffuse reflector.
Ordinary writing papers are good examples of diffuse reflectors, where as mirrors or highly polished metal plates are examples of specular reflectors. Almost all terrestrial surfaces (except calm water) act as diffuse reflectors of incident solar radiation.
diffuse sky radiation: Solar radiation reaching the earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. Also called skylight, diffuse skylight, sky radiation.
Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25 percent of the incident radiation), about two-thirds ultimately reaches the earth as diffuse sky radiation.
diffuse skylight: = diffuse sky radiation.
diffuse sound: Sound energy for which energy is uniform in the region considered and when all directions of energy flux at all parts of the region are equally probable.
diffuser: A specially designed duct, chamber, or section, sometimes equipped with guide vanes, that decreases the velocity of a fluid, as air, and increases its pressure, as in a jet engine, a wind tunnel, etc. See supersonic diffuser.
diffusers: Specially designed ducts, chambers, or sections, sometimes equipped with guide vanes, that decrease the velocity of a fluid, as air, and increases its pressure, as in jet engines, wind tunnels, etc. Used for shock diffusers.
diffusion: The interpenetration of one substance into another as a result of thermal / random motion of the individual particles.
diffusion: In an atmosphere, or in any gaseous system, the exchange of fluid parcels between regions, in apparently random motions of a scale too small to be treated by the equations of motion.
2. In materials, the movement of atoms of one material into the crystal lattice of an adjoining material, e.g.,penetration of the atoms in a ceramic coating into the lattice of the protected metal.
3. In ion engines, the migration of neutral atoms through a porous structure incident to ionization at the emitting surface.
diffusion coefficient: The absolute value of the ratio of the molecular flux per unit area to the concentration gradient of a gas diffusing through a gas or a porous medium where the molecular flux is evaluated across a surface perpendicular to the direction of the concentration gradient. See diffusion, coefficient of mutual diffusion.
diffusion equation: See diffusivity.
diffusion velocity: 1. The relative mean molecular velocity of a selected gas undergoing diffusion in a gaseous atmosphere, commonly taken as a nitrogen (N₂) atmosphere.
The diffusion velocity is a molecular phenomenon and depends upon the gaseous concentration as well as upon the pressure and temperature gradients present.
2. The velocity or speed with which a turbulent diffusion process proceeds as evidenced by the motion of individual eddies.
diffusive equilibrium: The steady state resulting from the diffusion process, primarily of interest when external forces or sources and sinks exist within the field. See isothermal equilibrium.
diffusivity: A measure of the rate of diffusion of a substance, expressed as the diffusivity coefficient K. When K is constant, the diffusion equation is where q is the substance diffused; ² is the Laplacian operator; and t is time. The diffusivity has dimensions of a length times a velocity; it varies with the property diffused, and for any given property it may be considered a constant or a function of temperature, space, etc., depending on the context. Also called coefficient of diffusion. See conductivity, kinematic viscosity, exchange coefficients.
In the case of molecular diffusion the length dimension is the mean free path of the molecules. By analogy, in eddy diffusion, length becomes the mixing length. The coefficient is the called then eddy diffusivity, and is in general several orders of magnitude larger than the molecular diffusivity.
difluence: The divergence vector of adjacent streamlines. The opposite of confluence.
dig pole: = magnetic pole.
digit: 1. A single symbol or character representing an integral quantity.
2. Any one of the symbols used in positional notation as coefficients of each power, or order, of the base. See binary digit, decimal digit.
digital: Using discrete expressions to represent variables.
digital computer: A computer which operates with information, numerical or otherwise, represented in a digital form.
digital count: Digital count is the total number of pixels occurring in an image for each possible data value.
digital data: Data in numerical form that can be stored and used in a computer.
digital electronics: The use of circuits in which there are usually only two states possible at any point. The two states can represent any of a variety of binary digits (bits) of information. Used for digital circuits.
digital filters: Computational means of attenuating undesired frequencies in sets of time-dependent data.
digital output: Transducer output that represents the magnitude of the stimulus in the form of a series of discrete quantities coded to represent digits in a system of notation. Compare analog output.
digital signals: Data that is converted to numerical form so that it can be stored and used in a computer.
digital television: Television in which picture redundancy is reduced or eliminated by transmitting only the data needed to define motion in the picture, as represented by changes in the areas of continuous white or black.
digitization: Digitization of graphics typically involves recording the location of each point, line, and polygon on a map, along with their associated labels and attributes.
digitize: To express an analog measurement of a variable in discrete units.
digitizer: A device which converts analog data into numbers expressed in digits in a system of notation. Also called analog-to-digital converter.
dihedral angle: The acute angle between two intersecting planes or between lines representative of planes.
dike: Dikes are tabular or sheet-like bodies of magma that cut through and across the layering of adjacent rocks. They form when magma rises into an existing fracture, or creates a new crack by forcing its way through existing rock, and then solidifies. Hundreds of dikes can invad the cone and inner core of a volcano, sometimes preferentially along zones of structural weakness.
Dione: A satellite of Saturn orbiting at a mean distance of 378,000 kilometers.
dioptric light: A light concentrated into a collimated beam by means of refracting lenses or prisms.
One collimated by means of a reflector is a catoptric light.
dip: 1. = dip angle.
2. = magnetic dip.
dip angle: (symbol )
The vertical angle between the true horizon and the apparent horizon.
dip equator: = aclinic line.
diplex transmission: The simultaneous transmission of two signals using a common carrier wave. Compare duplex operation, multiplexing.
diplexer: A device permitting an antenna system to be used simultaneously or separated by two transmitters. Compare duplexer.
dipole: 1. A system composed of two, separated, equal electric or magnetic charges of opposite sign.
2. = dipole antenna.
dipole antenna: A straight radiator, usually fed in the center, and producing a maximum of radiation in the plane normal to its axis. The length specified is the overall length.
Common usage in microwave antennas considers a dipole to be a metal radiating structure which supports a line current distribution similar to that of a thin straight wire, a half wavelength long, so energized that the current has two nodes, one at each of the far ends.
dipoles: Systems composed of two, separated, equal electric or magnetic charges of opposite sign.
direct air cycle: A thermodynamic propulsion cycle involving a nuclear reactor and gas turbine or ramjet engine, in which air is the working fluid. Also called direct cycle.
Air is successively compressed in the compressor section, heated in the nuclear reactor, and expelled through the turbine-tailpipe section to obtain thrust.
direct broadcast satellites: Domestic satellites used for direct TV transmission to home receivers. Used for DBS (satellites).
direct current: Electric current which travels continuously in the same direction over a sustained period of time; contrast with AC (Alternating Current) which oscillates as a function of time.
direct cycle: = direct air cycle.
direct motion: Eastward or counterclockwise motion of a planet or other object as seen from the North Pole (motion in the direction of increasing right ascension).
direct product: = scalar product.
direct solar radiation: In actinometry, that portion of the radiant energy received at the instrument direct from the sun, as distinguished from diffuse sky radiation, effective terrestrial radiation, or radiation from any other source. See global radiation.
Direct solar radiation is measured by pyrheliometers.
direct wave: A radio wave which travels directly from the transmitting antenna to the receiving antenna, in contrast with an indirect wave, which undergoes an abrupt change of direction by refraction or reflection.
direct-current discharge: The conduction of direct current through two electrodes immersed in a gas. See Townsend discharge, glow discharge, arc discharge.
direction angle: In tracking, the angle between the antenna base line and an imaginary line connecting the center of the baseline with the target.
direction cosine: 1. The cosine of the angle formed by the intersection of a line, as a line of sight to an orbiting body, with an axis of a rectangular coordinate system with the origin on the line.
Every line has three direction angles and three direction cosines: 1, m, n corresponding to , , , the direction angles with the x, y, and z axes.
2. Specifically, in tracking, the cosine of the angle between a base line and the line connecting the center of the baseline with the target.
direction finder: (abbr DF)
direction finding: A procedure or process for locating or localizing the origin of radar, acoustical, or optical emissions.
direction of relative movement: See relative movement.
directional antenna: An antenna that radiates or receives radio signals more efficiently in some directions than in others. See Adcock antenna, loop antenna, sense antenna.
A group of antennas arranged for this purpose is called an antenna array.
directional array: = antenna array.
directional emittance: Emittance in a stated direction from a surface. The direction is usually specified as angle from the normal.
directional gyro: 1. A two-degree-of-freedom gyro with a provision for maintaining its spin axis approximately horizontal.
2. A flight instrument incorporating a gyro that holds its position in azimuth and thus can be used as a directional reference.
directional properties: 1. Of metals, properties whose magnitude varies depending on the relation of the test axis to a specific direction within the metal. The variation results from preferred orientation or from fibering of constituents or inclusions.
2. For thermal radiation properties, in a specified direction from the surface, usually measured as the angle from the normal.
directional solidification (crystals): Controlled solidification (crystal growth) of molten metal in a casting so as to provide feed metal to the solidifying front of the casting.
directional stability: The property of an aircraft, rocket, etc., enabling it to restore itself from a yawing or sideslipping condition. Also called weathercock stability.
directivity: The ability of an antenna to radiate or receive more energy in some directions than in others. See beam.
The directivity of an antenna implies a maximum value, and it is equal to the ratio of the maximum field intensity to the average field intensity at a given distance.
directories: Alphabetical, geographical, or classified listings by field of persons, organizations, programs and/or objects such as instruments, devices, and products. Use of this term excludes directories in computers.
directrix: An auxiliary line used in the geometrical construction of a conic.
disasters: Large-scale drought, glacier movement, floods, fires, storms, etc.
discharge correction factor: (symbol _d)
Of a rocket nozzle, the ratio of the mass flow rate in the nozzle to that of an ideal nozzle which expands an identical working fluid from the same initial conditions to the same exit pressure.
discharge tube: A form of cold-cathode ionization gage in which the color and form of a cold-cathode discharge, without the presence of a magnetic field, gives an indication of the pressure and the nature of the gas.
discone antenna: An antenna formed of a disk and a cone whose apex approaches and becomes common with the outer conductor or the coaxial feed at its extremity.
The center conductor terminates at the center of the disk which is perpendicular to the axis of the cone.
discontinuity: A break in sequence or continuity of anything.
discontinuity: A break in sequence or continuity of anything.
Discos (satellite attitude control): A satellite orbit "DIsturbance COmpensation System" designed to maintain an object (proof object) in correct orbit by detecting forces and compensating for them by using thrusters.
discrete: Composed of distinct or discontinuous elements.
discrete address beacon system: Radar beacon system with discretely addressable transponders and a ground-air-ground data link for automated air traffic control (FAA).
discrete aurora: Typical ribbon-like structures of aurora observed from the ground. From space they may appear as brighter spots in the diffuse aurora.
discrete radio source: A source of small angular extent of cosmic radio waves.
discrete spectrum: A spectrum in which the component wavelengths (and wave numbers and frequencies) constitute a discrete sequence of values (finite or infinite in number) rather than a continuum of values. See continuous spectrum.
discrete variable: A quantity that may assume any one of a number of individually distinct or separate values.
discriminant analysis (statistics): A linear combination of a set of N variables that will classify (into two different classes) the events or items for which the measurements of the N variables are available, with the smallest proportion of misclassifications. Used for discriminant functions.
discriminator: In general, a circuit in which output depends upon the difference between an input signal and a reference signal.
dish: A parabolic reflector type of radio or radar antenna.
disk: The visible surface of the Sun (or any heavenly body) projected against the sky.
disk galaxies: Galaxies consisting of a central bulge of a spheroidal aggregation of stars and a surrounding disk of stars fanning outward in a thin layer.
disk operating system (DOS): A program with which the computer performs such mundane but useful tasks as storing, locating, and retrieving files on disk, reading the keyboard, and issuing display and print information.
dislocation: In crystallography, a type of lattice imperfection whose existence in metals is postulated in order to account for the phenomenon of crystal growth and of slip, particularly for the low value of shear stress required to initiate slip.
One section of the crystal adjacent to the slip plane is assumed to contain one more atomic plane that the section on the opposite side of the slip plane. Motion of the dislocation results in displacement of one of the sections with respect to another.
dispersion: 1. In rocketry (a) deviation from a prescribed flight path, (b) specifically, circular dispersion.
2. A measure of the scatter of data points around a mean value or around a regression curve.
Usually expressed as a standard-deviation estimate, or as a standard error of estimate. Note that the scatter is not centered around the true value unless systematic errors are zero.
3. The process in which radiation is separated into its component wavelengths.
Dispersion results when an optical process, such as diffraction, refraction, or scattering, varies according to wavelength.
4. In spectroscopy, a measure of the resolving power of a spectroscope or spectrograph, usually expressed in angstroms per millimeter.
5. As applied to materials, a scattering of very fine particles (e.g., ceramics) within the body of a metallic material usually resulting in overall strengthening of the composite material.
dispersion equation: = frequency equation.
dispersive medium: A medium in which the phase velocity of a wave, either electromagnetic or hydromagnetic, is a function of the frequency.
A plasma is a dispersive medium whereas free space is not, since waves of all frequencies travel in free space with the velocity of light.
displacement: A vector quantity that specifies the change of position of a body or particle usually measured from the mean position or position of rest.
Displacement can be represented by a rotation vector or translation vector or both.
displacement manometer: A differential manometer which indicates the pressure difference, if any, across a solid or liquid partition which can be displaced against a restoring force.
display: The graphic presentation of the output data of any device or system.
dissociation: The separation of a complex molecule into constituents by collision with a second body, or by absorption of a photon.
The product of dissociation of a molecule is two ions, one positively charged and one negatively charged.
dissociative recombination:: The combination of an electron with a positive molecular ion, followed by dissociation of the molecule in which the resulting atoms/molecules carry off the excess energy released in the recombination.
dissolved gases: Gases in solution.
distance marker: A reference marker indicating distance, particularly such a marker on a radar indicator, to indicate distance of a target from the radar antenna. On a plan position indicator it is usually one of a series of concentric circles. Also called range marker. See range ring.
distance measuring equipment: (abbr DME)
A radio aid to navigation which provides distance information by measuring total round-trip time of transmission from an interrogator to a transponder and return.
distorted-angle fabric: Material of a special, often basketlike weave suitable for pressure suits. Such fabrics permit a certain amount of flexibility when the suit is pressurized.
distortion: 1. An undesired change in waveform.
Noise and certain desired changes in waveform, such as those resulting from modulation or detection, are not usually classed as distortion.
2. In a system used for transmission or reproduction of sound, a failure by the system to transmit or reproduce a received waveform with exactness.
3. An undesired change in the dimensions or shape of a structure as, distortion of a fuel tank due to abnormal stresses or extreme temperature gradients.
distributed feedback lasers: Lasers containing a periodic medium which provides the necessary feedback for laser action.
distributed processing: Processing with multiple small computers that are capable of operating independently but can communicate over a network with each other and/or a central computer.
distribution function: The density function or number of particles per unit volume of phase space. The distribution function is a function of the three space coordinates and the three velocity coordinates.
A point in phase space represents a given position in ordinary space and a given velocity in velocity s

Aerospace Science & Technology Dictionary

Aerospace Science
&
Technology Dictionary