An Etymological Dictionary of Astronomy and Astrophysics

According to the → de Broglie hypothesis, which has been verified by experiments, every → particle of matter, whatever its nature, has a characteristic → wavelength
associated with its wavelike quantum aspect. The de Broglie equation gives the equivalent wavelength of a moving particle: λ = h/mv, where h is → Planck’s constant, m the mass of the particle, and v its velocity.

See also: → de Broglie wavelength, → Davisson-Germer experiment.

Etymology (EN): Named after Louis Victor de Broglie (1892-1987), French physicist, creator of a new field in physics, wave mechanics, who won the Nobel prize in physics in 1929. → equation

The suggestion by Louis de Broglie in 1924 whereby if → electromagnetic waves possess particle properties (→ particle nature), then it might be reasonable to suppose that material particles, such as → electrons, should possess wave properties (→ wave nature). The de Broglie hypothesis was based on the intuitive feeling that nature seems to have strong attachment to symmetry. In other words,
if radiation has particle-like properties, then material particles should possess wave-like properties. At the time no direct experimental evidence was present for the validity of this suggestion. The first confirmation of de Broglie’s hypothesis was provided by the → Davisson-Germer experiment. See also → wave-particle duality;

See also: → de Broglie equation; → hypothesis.

The wavelength of the wave associated with a → particle as given by the → de Broglie equation.

See also: → de Broglie equation; → wavelength.

A solution to → Einstein’s field equations of → general relativity which contains no ordinary matter (Ω_M = 0) or radiation (Ω_R = 0), is → Euclidean (k = 0), but has a → cosmological constant (Ω_Λ > 0). The Universe expands exponentially forever. This solution was the first model expanding of → expanding Universe. See also → empty Universe, → Milne Universe.

See also: After the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → universe.

A mathematical expression describing the → surface brightness profile of a typical → elliptical galaxy,
whereby the projected luminosity distribution varies as exp(-R)^1/4.

See also: → de Vaucouleurs radius; → law.

An → isophotal radius of a galaxy, where the → surface brightness in the B band falls to 25 mag arcsec^-2.

See also: After the French-born American astronomer Gérard de Vaucouleurs (1918-1995); → radius.

A widely used scheme for classifying galaxies which is an extension to the → Hubble sequence Hubble sequence. De Vaucouleurs introduced a more elaborate classification system for spiral galaxies (→ spiral galaxy) based on three morphological characteristics:

1. Bars. Galaxies are divided on the basis of the presence or absence of a nuclear → stellar bar. SA denotes spiral galaxies without bars, complementing Hubble’s use of SB for → barred spirals. De Vaucouleurs also defined an intermediate class, SAB, containing weakly barred spirals. → Lenticular galaxies are also classified as unbarred (SA0) or barred (SB0), with the notation S0 reserved for those galaxies for which it is impossible to tell if a bar is present or not.
   

2. Rings. Galaxies possessing ring-like structures are denoted (r), those without rings (s), and transition galaxies (rs).
   

3. Spiral arms. These are divided into the following subclasses: Sd (SBd), that is diffuse, broken arms made up of individual stellar clusters and nebulae; very faint central → bulge;

Sm (SBm): irregular in appearance; no bulge component; and Im, that is highly irregular galaxy.

Galaxies in the classes Sm and Im are termed the → Magellanic spirals and irregulars, respectively. The → Large Magellanic Cloud is of type SBm, while the → Small Magellanic Cloud is an irregular (Im).

See also: → de Vaucouleurs radius; → system.

A prefix occurring in loanwords from L. to indicate:

1. Opposition, reversal: decentralize, decode, deactivate, de-emphasis.
   
2. Removal from: dethrone.
   
3. Reduction, degradation: declass, devalue.
   
4. reversal: detract, detect.
   
5. Derivation from: derive, deverbative.

Etymology (EN): M.E., from O.Fr. de-, des-, partly from L. de- “from, down, away,” and partly from L. → dis-.

Etymology (PE): Prefix vâ- denoting “reversal, opposition; separation; repetition; open; off; away,” variant of bâz-, from Mid.Pers. abâz-, apâc-; O.Pers. apa- [pref.] “away, from;” Av. apa- [pref.] “away, from,”
apaš [adv.] “toward the back;” cf. Skt. ápāñc “situated behind.”
Foru- “down, downward; below; beneath,” Mid.Pers. frôt “down, downward,” O.Pers. fravata “forward, downward,” Skt. pravát- “a sloping path, the slope of a mountain.”

Transition from an excited energy state to a lower energy level, as in spectral line formation or particle emission from an atomic nucleus.

See also: From → de- + → excitation.

To cause a spacecraft to leave its operational orbit to enter a descent phase or to change course.

See also: From → de- + → orbit.

Of a spacecraft, the act or process of departing from an operational orbit. → de-orbit.

See also: Noun form of → de-orbit.

A method for estimating the real orientation of a field and/or related velocities/separations from two-dimensional images.

See also: → de-; → projection.

To correct the intensity or flux of a spectral line by eliminating the → reddening effect of → interstellar dust.

Etymology (EN): De-redden, from → de- + redden verb of
→ red.

Etymology (PE): Vâsorxidan, from vâ-, → de-, + sorxidan verb of sorx, → red.

A magnitude which has been corrected for the interstellar reddening.

Etymology (EN): De-reddened, p.p. of → de-redden;
→ magnitude.

Etymology (PE): Borz, → magnitude; vâsorxidé p.p. of vâsorxidan, → de-redden.

The process that de-reddens. The state of being de-reddened.

See also: Verbal noun of → de-redden.

To cause to be → inactive; remove the → effectiveness of (Dictionary.com).

See also: → de-; → activate.

1. Deprived of life.
   

2. No longer producing or functioning. → dead pixel.

Etymology (EN): M.E. deed, O.E. dead “dead;” cf. O.S. dod, Dan. død, Swed. död, Du. dood, O.H.G. tot, Ger. tot; PIE *dhou-toz-, from base *dheu- “to die.”

Etymology (PE): Mordé “dead,” p.p. of mordan, mir- “to die,” → death.

Of a → CCD detector, a pixel that is not sufficiently active.

See also: → dead; → pixel.

The time by which something must be finished or submitted; the latest time for finishing something (Dictionary.com).

Etymology (EN): Historically, during the American Civil War, the term referred to a physical line or boundary beyond which prisoners were shot; → dead; → line.

Etymology (PE): Ruzband, literally “day lock, that which blocks date,” from ruz, → day, + band, → bond; cf. Ger. Stichtag, Anmeldeschluss, Einsendeschluss and Fr. date de clôture.

The act of dying; the end of life.

Etymology (EN): M.E. deeth, O.E. deað (cf. O.S. doth, O.Fris. dath, Du. dood, O.H.G. tod, Ger. Tod, O.N. dauði, Dan. død, Swed. död, Goth. dauþas “death”), from dheu- “to die” + -thuz “-th;”

Etymology (PE): Marg “death,” from Mid.Pers. marg “death;” Av. mahrka- “death,” mərəxš- “ruin;” related to Mod.Pers. mordan (present stem mir-) “to die;” Mid.Pers. murdan
“to die;” O.Pers. mrt- “to die,” amriyta “dies,” martiya- “(mortal) man” (Mod.Pers. mard “man”); Av. mərəta- “died;” cf. Skt. mar- “to die,” mrti- “death,” marc- “to damage, hurt, injure;” PIE base *mor-/*mr- “to die.” Cognates in other IE languages: Gk. emorten “died,” ambrotos “immortal;”
L. morior “I die,” mortuus “dead” (Fr. mourir “to die,” mort “dead”); Arm. merani- “to die;” O.C.S. mrutvu “dead;” O.Ir. marb; Welsh marw “died;” O.E. morþ “murder;” Lith. mirtis.

1a) A → discussion, as of a public question in an assembly, involving opposing viewpoints.

1b) A formal contest in which the affirmative and negative sides of a proposition are advocated by opposing speakers.

2. To engage in argument or discussion, as in a legislative or public assembly (Dictionary.com).

Etymology (EN): M.E. debaten, from O.Fr. debatre, literally “to fight,” from → de- “down, completely,” + batre “to beat,” → beat.

Etymology (PE): Vâzat, from vâ-, → de-, + zat- (Baluci žat-) variant of zad-, zadan “to strike, beat”, → beat.

To substract the → bias from a flat-field or science frame obtained using an electronic detector.

Etymology (EN): Debias, from → de- + → bias.

Etymology (PE): Varak zodudan, from varak→ bias + zodudan “to polish, clean,” Mid.Pers. uzdātan, Av. uzdā-, from uz-, → ex-, + dā- “make, create.”

The process of removing the → bias from a → flat-field or science → frame. → debias

See also: Verbal noun of → debias.

In → spectroscopy, to → resolve a → blend into its constituent → spectral lines.

Etymology (EN): Deblend, from → de- + → blend.

Etymology (PE): Tuham, → blend, + zodudan “to polish, clean,” Mid.Pers. uzdātan, Av. uzdā-, from uz-,
→ ex-, + dā- “make, create.”

The process or state of separating or resolving into constituent → spectral lines.

See also: Verbal noun of deblend, from → de- + → blend.

To → increase the → spatial resolution or the clearness of an → image. Same as → sharpen. See also → deconvolve.

Etymology (EN): Deblur, from → de- + → blur.

Etymology (PE): Tigidan “to sharpen,” from tig “sharp,” variants tiz, tež, tej, tij, tiq, Mid.Pers. tigr, têz, têž “sharp,” O.Pers. tigra- “pointed,”
tigra.xauda- “pointed helmet (epithet of Scythians),” Av. tiγra- “pointed,” tiγray- “arrow,” tiži.arštay- “with the pointed spear,” cf. Skt. tikta- “sharp, pungent, bitter,” tejas- “sharpness, edge, point or top of a flame;” PIE base *st(e)ig- “to stick; pointed.” Cognates in other IE languages: Gk. stizein “to prick, puncture,” stigma “mark made by a pointed instrument,” L. in-stigare “to goad,” O.H.G. stehhan, Ger. stechen “to stab, prick,” Du. stecken, O.E. sticca “rod, twig, spoon,” E. stick.

The process or state of making an image sharper. → deblur; same as sharpening. → deconvolution.

See also: Verbal noun of → deblur.

The remains of anything broken down or destroyed; rubble; ruins.
Astro.: → debris disk; → orbital debris.
Geology: An accumulation of loose fragments of rock.

Etymology (EN): From Fr. débris, from M.Fr. débriser “break down, crush,” from O.Fr. debrisier, → de- + brisier “to break,” from L.L. brisare.

Etymology (PE): Tifâl, from tif “rubbish, sweepings, debris” + -al relation suffix → -al.

A disk developing around a star after the dissipation of the → protoplanetary disk of gas and dust whose material was used in the formation of planets during the first 10 million years. The resulting debris disk is mainly composed of residual → planetesimals
analogous to → asteroids, → comets, and → Kuiper Belt Objects in the Solar System. Their mutual collisions produce observable → dust emission in a belt encompassing the planetary system.

See also: → debris; → disk.

A → unit of electric → dipole moments, equal to 10^-18 → CGS units or 3.336 x 10^-30 → coulomb meter.

Etymology (EN): After Peter Joseph Wilhelm Debye (1884-1966), Dutch-born American, who made important studies in the conductivity of electricity by salt solutions and in the heat capacity of solids. He received the 1936 Nobel Prize in chemistry for his work on the structure of molecules.

The → Debye model at → low  → temperatures, where the → specific heat is proportional to the cube of the → absolute temperature, i.e. C_V ∝ T³.

See also: → debye; → law.

A characteristic length scale in a → plasma, determined by the temperature and number density of the charged particles. The Debye length (in cm) is given by the expression:
λ_D = 743(T_e/n_e)^1/2, where T_e is the electron temperature in → electron-volts
and n_e is the electron density in cm^-3. Particles which pass each other at distances smaller than the Debye length interact directly. Outside the Debye length particle interactions are dominated by collective effects. In typical laboratory plasmas the Debye length is small compared with the plasma dimension, so that collective effects are important.

See also: → debye (D); → length.

An extension of the → Einstein model accounting for → specific heats, based on the concept of → elastic waves in → crystals. In this model specific heat is given by: C_V = 9R[(4/x²)∫ y²/(e^y - 1)dy - x/(e^x - 1)], integrating from 0 to x, where R is the → gas constant, k is → Boltzmann’s constant, x = hν_max/k, and y = hν/k. The parameter T_D = hν_max/k is the characteristic → Debye temperature of the crystal. At low temperatures the specific heat prediction by this model is in good agreement with observations (→ Debye law),
in contrast to Einstein’s model.

See also: → debye; → temperature.

The characteristic → temperature of the → crystal as given by the → Debye model of → specific heats.

See also: → debye; → temperature.

Prefix meaning “ten” and “ten times” used in terms belonging to the metric system.

Etymology (EN): From Fr. déca-, from L. deca-, dec-, from Gk. deka “ten;”
cf. Av. dasa “ten,” Mod.Pers. dah “ten,” Skt. dasa, PIE *dekm (Dan. ti, Du. tien, Ger. zehn, E. ten, Fr. dix).

Etymology (PE): Dekâ-, from Fr. as above.

A ten-sided → polygon. A decagon that is equilateral and equiangular is called a regular decagon. Each angle of a regular decagon is 144°. The central angle subtending any side is 36°. The ratio of the radius to the side of a regular decagon forms the → golden ratio.

See also: → deca-; → -gon.

1a) To become decomposed.

1b) Of a radioactive nucleus, to disintegrate spontaneously into one or more different nuclei, accompanied by the emission of → alpha particles, → beta particles, → positrons, and/or → gamma rays.

2a) Decomposition.

2b) The → transition of a system from an → excited state to a less excited one. Radiative decay refers to the process when the energy difference between the states is taken away by radiation. Likewise, collisional decay is when the energy difference is removed by a target during a collision.

2c) Of a spacecraft, a gradual decrease in the radius of its orbit over time, caused by aerodynamic drag of the atmosphere and other forces. The rate of orbit decay rises as the spacecraft falls and encounters increasing atmospheric density, eventually resulting in reentry.
→ alpha decay; → beta decay; → dark matter decay; → decay chain; → decay constant; → decay mode; → decay product; → decay time; → inverse beta decay; → Ohmic decay time; → orbit decay; → radioactive decay; → turbulence decay.

Etymology (EN): From O.Fr. decair, from V.L. *decadere “to fall off,” from L. cadere “to fall,” PIE base *kad- “to fall” (cf. Pers. Gilaki katan “to fall,” ba.ka.tam “I fell,” dakatan “to fall (in a marsh, in a pit),” vakatan “to fall from tiredness, be exhausted,” fakatan “to fall from (lose) reputation,” Pers. Laki: katen “to fall,” kat “he fell,” beko “fall!,” Pers. Tabari: dakətə “fallen,” dakətən “to crash down,” dakət.gu “stray cow,” Arm. chacnum “to fall”).

Etymology (PE): Tabâhidan, verbal form of tabâhi, noun form of tabâh “spoiled, ruined, destroyed,” Mid.Pers. tapâh “spoiled, destroyed.” Maybe related to Mod.Pers. tâb “affliction, pain, torment; heat, burning,” tab “fever,” tâbidan, tâftan “to shine,” tafsidan “to become hot,” Av. tāp-, taf- “to warm up, heat,” tafsat “became hot,” tāpaiieiti “to create warmth,” cf.
Skt. tap- “to spoil, injure, damage; to suffer; to heat, be/become hot,” tapati “burns,” L. tepere “to be warm,” tepidus “warm,” PIE base *tep- “warm.”

A series of nuclear decays produced by successive → daughter products, when the daughters are themselves → radioactive. For example, the decay chain N₁→ N₂→ N₃→ … in which the parent nuclide N₁ decays to the daughter N₂, which in turn decays to N₃. Each → radionuclide in the decay chain can → branch to more than one daughter.

See also: → decay; → chain.

A constant of proportionality occurring in the formula expressing spontaneous → decay of → radionuclides. The number of atoms decaying is given by N = N₀e^-kt, where N₀ is the number of nuclei in the given volume of the substance at instant t = 0, N is the number of nuclei at t, and k is decay constant. Decay constant is related to → half-life by τ = ln2/k, roughly 0.693/k.

See also: → decay; → constant.

A possible type of decay of a → radionuclide or → elementary particle.
In general, a given particle may decay by more than one decay mode. Usually the number of decay modes is one or two. There are, however, → nuclides
which have many decay modes; for example the nuclide ¹¹Li has seven decay modes.

See also: → decay; → mode.

A → stable nuclide or → radioactive nuclide formed by the → disintegration of a → radioactive isotope, either directly or as a result of a → decay chain. Also called → daughter product. For example, the decay product of ²³⁸U is ²⁰⁶Pb, after passing through the following chain:

²³⁸U → ²³⁴Th (4.5 billion yr) → ²³⁴Pa (24 days) → ²³⁴U (1 min) → ²³⁰Th (245,000 yr) → ²²⁶Ra (76,000 yr) → ²²²Rn (1,600 yr) → ²¹⁸Po (3.8 d) → ²¹⁴Pb (3 m) → ²¹⁴Bi (27 m) → ²¹⁴Po (160 microseconds) → ²¹⁰Pb (22 yr) → ²¹⁰Bi (5 d) → ²¹⁰Po (138 d) → ²⁰⁶Pb.

See also: → decay; → product.

The time required for the amplitude of a vibrating system to decrease to 1/e of its initial value.
Of an orbiting object, its lifetime in a non stable orbit.

See also: → decay; → time.

(v.intr.) To slow down. (v.tr.) To decrease the velocity of.

Etymology (EN): From → de- + (ac)celerate, from
→ accelerate.

Etymology (PE): Vâšetâbidan, from vâ-→ de- + šetâbidan, → accelerate.

The act or process of moving, or of causing to move, with decreasing speed. Sometimes called negative acceleration.

See also: Verbal noun of decelerate.

A parameter designating the rate at which the expansion of the Universe would slow down owing to the braking gravitational effect of the matter content of the Universe. It is expressed by:
q(t) = -R(t)R ^..(t)/R ^.2(t), where R(t) represents the size of the Universe at time t. See also
→ expansion parameter; compare with → acceleration parameter.

See also: → deceleration;
→ parameter.

Prefix used in the metric system to mean on-tenth.

Etymology (EN): From Fr. déci-, from L. decimus “tenth,” from decem “ten,” cf. Av. dasa “ten,” Mod.Pers. dah “ten,” Gk. deka, Skt. dasa, PIE *dekm (Dan. ti, Du. tien, Ger. zehn, E. ten, Fr. dix).

Etymology (PE): Desi-, from Fr. déci-, as above.

A dimensionless unit used to express relative difference in power or intensity, usually between two acoustic or electric signals. It is defined as n [dB] = 10 log (P₁/P₀),
with → natural logarithm. One decibel equals one-tenth of a → bel (B).

See also: → deci-; → bel (B).

To make a choice or come to a conclusion about something. → decision, → rule of decision.

Etymology (EN): M.E. deciden, from O.Fr. decider, from L. decidere “to decide, determine,” literally “to cut off,” from → de-“off” + caedere “to cut, chop, beat, hew.”

Etymology (PE): Vâsunidan, from vâ- “off, away,” → de-,

• sun “to cut;” cf. Kurd. Soriani su, sun “to sharpen, whet;” Kurd. Kurmanji (prefixed ha-, contraction of *ham-) hasun “to sharpen, whet;” Mod.Pers. sân “whetstone,” variants fasân, afsân, awsân; Av. si-, sā- “to cut;” cf. Skt. śā- “to sharpen, whet;” + -idan infinitive suffix.

Someone who decides, or something that allows to determine.

See also: → decide; → -er.

Referring to or based in the number 10; relating to tens or tenths.

Etymology (EN): M.L. decimalis “of tithes or tenths,” from L. decimus “tenth,” from decem, → ten.

Etymology (PE): Dahi , of or relating to dah, → ten; dahdahi literally “ten by ten.”

A fraction expressed by using → decimal representation, as opposed to a vulgar fraction. For example, 2/5 is a vulgar fraction; 0.40 is a decimal fraction.

See also: → decimal; → fraction.

→ common logarithm.

See also: → decimal; → logarithm.

A system of numerals for representing real numbers that uses the → base 10. It includes the digits from 0 through 9.

See also: → decimal; → system.

The position of a digit to the right of a → decimal point written in decimal notation. In 0.032, for example, 0 is the first decimal place, 3 is the second decimal place, and 2 is the third decimal place.

Etymology (EN): → decimal; → place.

Etymology (PE): Raqam, → digit; pas, → after; jodâgar, momayez, → decimal point.

A symbol (usually a point or dot) used to separate the integer part from the fractional part of a → decimal number.

Etymology (EN): → decimal; → point.

Etymology (PE): Jodâgar, → separator.
Momayez, from Ar. mumaiyiz “distinguisher, discerner, separator.”

A representation of a real number between 0 and 1, as written a = d₁d₂d₃ …, where each d_i is one of the digits 0, 1, 2, …, 9. This means that a = d₁ x 10^-1 + d₂ x 10^-2 + d₃ x 10^-3+ …

See also: → decimal; → representation.

same as → decimal number system.

See also: → decimal; → number; → system.

An electromagnetic radio wave having wavelengths between 10 cm and 1m, corresponding to a frequency between 300 and 3,000 Mega Hertz. It is naturally emitted by various astronomical objects.

Etymology (EN): Decimetric, from → deci- + from Fr. métrique, → metric; → wave.

Etymology (PE): Mowj, → wave; desimetri, from décimétrique, as above.

The act or process of deciding. → rule of decision.

See also: Verbal noun of → decide.

Having the power or quality of deciding; putting an end to controversy; crucial or most important (Dictionary.com).

See also: → decide; → -ive.

To speak aloud in an oratorical manner; make a formal speech (Dictionary.com).

See also: → de-; → claim.

The act of declaring; announcement.

2. Something that is announced or proclaimed.

See also: Verbal noun of → declare.

1. To make known or state clearly, especially in explicit or formal terms.
   

2. To announce officially; proclaim (Dictionary.com).

Etymology (EN): M.E. declaren, from O.Fr. declarer “explain, elucidate,” from L. declarare “make clear, reveal, disclose, announce,” from → de- intensive prefix + clarare “clarify,” from clarus, → clear.

Etymology (PE): From Kurd. (Sorani) rûnâk, runâg “clear, bright,” rûn “bright, clear,” rûn kirdin “to explain,” variants of rowšanâ, rowšan, rušanâ, → bright, → day.

The → inflection of nouns, pronouns, or adjectives for case, number, and gender.

Etymology (EN): M.E. declenson, declynson (with suffix later assimilated to -sion), from O.Fr. declinaison, from L. declinatio,
→ declination

Etymology (PE): → declination.

1. Angular distance (symbol δ) from the celestial equator. One of the coordinates, with → right ascension, that defines the position of a heavenly body in the equator system. Declination is measured positively (+) north or negatively (-) south of the celestial equator from 0° to 90°.
   
2. Angle between geographic North and magnetic North (symbol D).

See also: Verbal noun of → decline; → -tion.

The axis to which the telescope tube is fastened in an → equatorial mounting.

See also: → declination; → axis.

For a telescope with an → equatorial mounting, a graduated circle attached to the → declination axis that shows the → declination to which the telescope is pointing.

See also: → declination; → circle.

1a) To bend down; slope downward; descend.

1b) Grammar: To → inflect according to the various cases of a noun, adjective, or pronoun.

2. The process of declining; a downward movement; gradual loss or deterioration.

Etymology (EN): M.E. declinen, from O.Fr. decliner “to sink, decline, degenerate, turn aside,” from declinare
“to turn away, inflect,” from → de- + clinare “to bend,” cf. Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” Mod.Pers. kil “bent, inclined, slant,” PIE base *klei- “to lean, incline” from which is also derived
(Ger. lehnen, E. lean).

Etymology (PE): Vâkil, from prefix vâ- “away” + kil “bent, crooked, inclined” [Dehxodâ], Mid.Pers. xwahl “bent, crooked” (Mod.Pers. dialectal/colloquial variants kowleh, cowleh, cowl, caval, xohl, xohal, hol), cf. Skt. kûla- “slope, declivity;” PIE base *klei-, as above. → inclination

To convert encoded symbols so that they can be used by the computer or understood by a person. See also → encode and → decrypt.

Etymology (EN): → de-; → code.

Etymology (PE): Vâramzidan, from vâ-, → de-. Ramz bardâštan, from ramz, → code,

• bardâštan “to remove, to take,” from bar- “on; up; upon; in; into; at; forth; with,” → on-, + dâštan “to have; to hold; to posses,” → property.

An electronic device that converts audio or video signals into a different form that can be heard or seen correctly.

See also: → decode; → -er.

The process of translating data or a message from a code into the original language or form. See also → encoding and → decryption.

See also: → decode; → -ing.

Of a physical force or entity or chemical substance, to separate or break into constituent parts or elements.

See also: From → de- + → compose.

The process or the state of breaking down a physical entity or an organic material.

See also: From → de- + → composition.

To cause to undergo → decompression.

See also: → de- + → compress.

1. A gradual decrease in surrounding → pressure, especially the controlled decrease in pressure that divers undergo after working in deep water or breathing compressed air.
   
2. Computers: The restoration of compressed electronic data to their full size.

See also: → de- + → compression.

A mathematical operation that allows to restore the original input signal, such as an astronomical image or spectrum, to its state before being affected by the → atmospheric turbulence and the → transfer function of the instrument. → convolution.

See also: From → de- + → convolution.

An algorithm used to improve the resolution of an image particularly when the convolving function is well defined. Also called deconvolution code.

See also: → deconvolution; → algorithm.

Verbal form of → deconvolution.

See also: From → de- + → convolve.

1. To separate or detach; to cause to become disconnected or divergent.
   

2. Electronics: To reduce or eliminate the coupling of (one circuit or part to another).
   

3. Physics: To decrease or eliminate the shock waves of a nuclear explosion by having it take place underground.

See also: → de- + → couple.

Cosmology: In the early history of the Universe, separation of matter and radiation due to their non-interaction. At a redshift of 1000, that is about 400,000 years after the Big Bang, the temperature would have cooled to the point (4000 degrees Kelvin) where electrons and nuclei can recombine and form neutral hydrogen atoms. Since atoms do not scatter the radiation appreciably, free electrons were lacking, and the Universe became transparent to radiation. Same as → recombination.
See also → decoupling era.

See also: Verbal noun of decouple, from → de- + → couple + → -ing.

The era some 400,000 years after the → Big Bang, when the cosmic → blackbody radiation was last scattered by the matter. → decoupling.
Same as → recombination era and → last scattering epoch.

See also: → decoupling; → era.

1. (v.tr.) To cause to diminish, to make less. (v.intr.) To diminish or lessen in extent, quantity, power, etc.
   

2. The act or process of decreasing; condition of being decreased. → Forbush decrease.

Etymology (EN): Decrease, from M.E. decres (n.), decresen, from
O.Fr. descreistre, from L. decrescere, from → de- + crescere “to grow,” → crescent.

Etymology (PE): Kâstan, kâh-, from Mid.Pers. kâhitan, kâstan, kâhênitan “to decrease, diminish, lessen,” Av. kasu- “small, little” (Mod.Pers. keh), Proto-Iranian *kas- “to be small, diminish, lessen;” kâheš verbal noun from kâhidan.

1. The amount lost in the process of decreasing.
   

2. Math.: The quantity by which a variable is decreased. A negative → increment.
   

3. Physics: 1) The ratio of the amplitude of an oscillation to that of its succeeding oscillation in an underdamped vibrating system. 2) The intensity ratio of a series of spectral lines of the same nature, such as
   → Balmer decrement.

Etymology (EN): L decrementum, from decre(tus), → decrease + -mentum noun suffix -ment.

Etymology (PE): Kâheh, from kâh- present stem of kâhidan, → decrease + noun suffix -é.

A physical process that is contrary to accretion, referring to the outward direction of the mass flow from a star to its environment.

See also: From → de- “reversal, removal,” + -cretion, from L. accretionem (nom. accretio, gen. accretionis) “a growing larger,” from stem of accrescere, → accretion.

A disk that would form around a star when the star injects matter into a close orbit. This is in contrast to an → accretion disk, which transfers matter from outside to the star. The mass gets injected into the decretion disk by a not yet well-known mechanism, most probably a combination of → non-radial pulsation, fast rotation, and possibly small-scale → magnetic fields. See also → viscous decretion disk.

See also: → decretion; → disk.

Make crypted data or information intelligible. See also → encrypt and → decode.

See also: → de- + crypt, → encrypt.

The process of restoring encrypted data back to the original information. See also → encryption and → decoding.

See also: → decrypt; → -tion.

To derive as a conclusion from facts or premises.

Etymology (EN): L. deducere “to lead down, derive,” from → de- “down” + ducere “to lead.”

Etymology (PE): Foruhâxtan, foruhâzidan, from foru- “down,” → de- + Mid.Pers. hâxtan, hâzidan “to lead, guide, persuade,” Av. hak-, hacaiti “to attach oneself to, to join,” cf. Skt. sacate “accompanies, follows,” Gk. hepesthai “to follow,” L. sequi “to follow;” PIE *sekw- “to follow.”

1. The act or process of deducting; something that is or may be deduced.
   

2. A process of reasoning in which a conclusion
   is derived from the premises presented without a need for additional information. → deductive reasoning.

See also: Verbal noun from → deduce.

Of or relating to → deduction.

See also: From deduct, → deduce, + → -ive.

Reasoning from the → general to the → particular (or from → cause to → effect).

See also: → deductive; → reasoning.

1a) General: Extending well inward from an outer surface or back from an edge.

1b) Great in measure; intense. → deep exposure.

1c) Of colors, dark and vivid.

2. The deep part of a body of water, especially an area of the ocean floor having a depth greater than 5400 meters (Dictionary.com).

Etymology (EN): O.E. deop, from P.Gmc. *deupaz, from PIE *d(e)u- “deep, hollow.”

Etymology (PE): Žarf “deep;” variants Gilaki jalf, julf, jal; Tabari jol, jal, jul; Baluci jahl, johl; Kermâni jarr “deep;” Mid.Pers. zufr; Av. jafra- “deep.”
Gowd, probably ultimately from PIE root *g^wādh- “to sink, submerge;” cf. Av. vigāθô- “ravines, gorges;”
Skt. gādha- “depth; shallow;” Gk. bessa “gorge, ravine.”

An exposure in which the detector shutter remains open for a relatively long time in order to bring out the weaker features of the observed object. In practice a deep exposure with a CCD detector is usually obtained from co-addition of shorter exposures.

See also: → deep; → exposure.

An area on the sky whose image is obtained with a deep exposure, such as → Hubble Deep Field.

See also: → deep; → field.

An image obtained using a deep exposure to reveal the weak features of the object.

See also: → deep; → image.

The time-scale of geologic processes which is millions or billions of years in contrast to the few thousand years claimed by supporters of the → creationism.

The concept of “deep time” was first described in 1788 by the Scottish geologist James Hutton (1726-1797) in the Transactions of the Royal Society of Edinburgh. The term was coined by the American author John McPhee (1931-).

See also: → deep; → time.

1. Failure to act; inaction or neglect; failure to meet financial obligations.
   

   2. Lack; want; absence. See also → deficiency, → lack, → shortage.
      

   3. Computers: A selection automatically used by a program when none is specified by the user. → by default.

Etymology (EN): M.E. defau(l)te, O.Fr. defaute “fault, defect, failure,” from V.L. *defallita “a deficiency or failure,” p.p. of *defallere, from L. → de- “away” + fallere “to deceive, to cheat; to put wrong, to lead astray.”

Etymology (PE): Nâbun, literally “not-being, non-existence,” from nâ- negation prefix, → non-, + (Kurd.) bun “to be,” variants (Gazi, Yarani, Gurani, Semnâni) bu-, classical Pers. bov-, budan “to be, → exist.”

General: Something or a lack of something that results in incompleteness, inadequacy, or imperfection.
Crystals: A discontinuity in the arrangement of atoms, ions, or electrons.
Cosmology: → cosmic defect.

Etymology (EN): From L. defectus “failure,” from p.p. of deficere “to fail, desert,” from → de- “down, away” + facere “to do,” (cf. Fr. faire, Sp. hacer), from PIE base *dhe- “to put, to do” (cf. Av. dadaiti “he puts,” Skt. dadhati “puts, places,” Hitt. dai- “to place,” Gk. tithenai “to put.”

Etymology (PE): Âk “defect, blemish;” Mid.Pers. ak, âk “evil, harm;” Av. aka- “bad, wicked;” cf. Skt. aka- “pain , trouble.”
Kâst “loss,” from kâstan, kâhidan “to decrease, lessen, diminish,” from Mid.Pers. kâhitan, kâstan, kâhênitan “to decrease, diminish, lessen;” keh “small, little, slender;”
Av. kasu- “small, little;” Proto-Iranian *kas- “to be small, diminish, lessen.”

1. To ward off attack from; guard against assault or injury.
   

   2. To maintain by argument, evidence, etc.; uphold (Dictionary.com).

Etymology (EN): From → de- “from, away,” + -fendere “to strike, push,” → offend.

Etymology (PE): Padâfandidan, from padâfand, from prefix pad- “contrary to; against; opposing,” → counter-,

• âfand “strife, war,” → offend.

A person who defends someone or something.

See also: → defend; → -er.

1. Resistance against attack; protection.
   

   2. The defending of a cause or the like by speech, argument, etc. (Dictionary.com).

See also: Noun from → defend.

1. Serving to defend; protective; of or relating to defense.
   

   2. A position or attitude of defense (Dictionary.com).

See also: → defense; → -ive.

In Ptolemy’s geocentric model, the circle around the Earth in which the center of the → epicycle of a body was thought to move.

Etymology (EN): Deferent, from L. deferent-, pr.p. of deferre “to carry from or down, transfer” from → de- + ferre “to carry, bear,” from PIE *bher- “to bear, to carry, to take” (cf. O.Pers./Av. bar- “to bear, carry,” barəθre “to bear” (infinitive), Mod.Pers. bordan, bar- “to carry,”
Skt. bharati “he carries,” Gk. pherein “to carry”).

Etymology (PE): Falak-e hâmel, from Ar. falak “heaven; orbit, sphere” + hamil “carrier.”

In a CCD detector, phenomenon caused by charge traps or potential pockets, which prevent electrons from being released to the adjacent pixel; eventually the electrons may be released in a subsequent cycle.

Etymology (EN): Differed, p.p. of differ, from O.Fr. différer, from L. differre “set apart, put off, delay,” also “be different, differ,” → differ; → charge.

Etymology (PE): Bâr→ charge; bâzdâšté “prevented,” p.p. of bâzdâštan, from preventive suffix bâz- + dâštan “to hold, have, maintain,” Mid.Pers. dâštan, O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind,” Skt. dhar- “to hold, keep, preserve,” dharma- “law,”
Gk. thronos “elevated seat, throne,” L. firmus “firm, stable,” Lith. daryti “to make,” PIE *dher- “to hold, support.”

See also: Noun from → deficient.

1. Lacking some element or characteristic; defective.
   

   2. Insufficient; inadequate.

Etymology (EN): From L. deficientem, pr.p. of deficere “to desert, fail,” from → de- “down, away” + facere “to do, perform” + -ency a noun suffix, equivalent to → -ence.

Etymology (PE): Maxeš, from (Dehxodâ) max “missing; vanished; annihilated,” variant (Nâyini) mak “missing;” maybe ultimately from Proto-Ir. *maiH- “to fade, damage, to harm;” cf. Av. (+ *ui-) vīmiti- “loss, destruction;” O.Pers. mīθah- “damage, harm;” Mid.Pers. myh-g’r “damage, harm;” Sogdian ‘’m’yk “dirt, mixture,” ‘myq “taint, infection;” Ossetic mynæg, minæg “weak, dim, fading;” Skt. mayⁱ “to damage, to lessen;” Gk. minytho “to diminish, to lessen;” L. minuoere “to diminish, to lessen,” minor “smaller, less;” PIE root *meiH- “to lessen, diminish” (Cheung 2007).

1. To state or set forth the meaning of (a word, concept, etc.).
   

2. To explain or identify the nature or essential qualities of.
   

3. To fix or mark the boundaries or extent of.

Etymology (EN): From O.Fr. définir “to end, determine,” from L. definire “to limit, determine, explain,” from → de- + finire “to bound, limit,” from finis “boundary.”

Etymology (PE): Hedâridan, infinitive from Semnâni hedâr “boundary (between pieces of lands),” Tabari hedâri “boundary, border,” may be related to
Av. haêtav- “dam, dike,” Skt. setu- “a ridge of earth, bank, dike, dam, bridge.”

1. Clearly characterized or fixed.
   

2. Showing clearly the outline or profile or boundary.
   

3. Computer science: A variable having received a value prior to a given point in a program.

See also: P.p. of → define.

Clearly defined or determined; having fixed limits. → definite integral.

Etymology (EN): From L. definitus “limited, precise,” p.p. of definire,
→ define.

Etymology (PE): Hedârmand from hedâr, stem of hedârdan, → define, + -mand possession suffix.

An integral with upper and lower limits.

See also: → definite; → integral.

1. The act or process of establishing some value or condition with precision.
   

2. The formal statement of the meaning or significance of a word, concept, etc.
   

3. Optics: The clarity, fidelity, sharpness, and brilliancy of an image formed by an optical system.

See also: Verbal noun of → define.

Most reliable or complete (as of a text, author, criticism, study, judgement, or the like) that serves as a standard or reference point.

Etymology (EN): M.E., from O.Fr., from L. definitivus, from definit(us)→ definite + -ivus a suffix of adjectives.

Etymology (PE): Hedârâ, adj., from hedâr present stem of hedâridan→ define + adj. suffix -â.

To burn suddenly and violently with great heat and intense light. → deflagration.

Etymology (EN): From L. deflagratus, p.p. of deflagrare “to burn down,” from → de- + flag(rare) “to blaze, glow, burn” (L. fulgur “lightning;” PIE *bhleg- “to shine;” cf. Gk. phlegein “to burn, scorch,” Skt. bhárgas- “radiance, lustre, splendour,” O.E. blæc “black”) + -atus “-ate”

Etymology (PE): Taškaftidan, from taš “fire,” variant of âtaš→ fire + kaftidan “to explode,” → explode.

A rapid → chemical reaction in which the → output of → heat is enough to enable the reaction to proceed and be accelerated without input of heat from another source. The effect of a true deflagration under confinement is an → explosion.
See also: → detonation; → explosion.

See also: Verbal noun of → deflagrate.

To bend or turn aside; turn from a true course or straight line.

Etymology (EN): From L. deflectere “to bend down, turn aside,” from → de- “away” + flectere “to bend, turn.”

Etymology (PE): Vâcaftidan, vâcaftan, from vâcaft, from vâ- “away,” → de- + caft “bent, curved, crooked,” variants cafté, jafté, jaftâ.

The act or state of deflecting or the state of being deflected. Amount of deviation.
Electronics: In a cathode-ray tube, the bending, by a magnetic field, of the beam of electrons after the focused beam leaves the electron gun.

See also: Verbal noun of → deflect.

1. The angle by which a light ray is curved by the gravitational field of a massive body. Einstein’s general relativity predicts that a light ray which tangentially grazes the surface of the Sun is deflected by 1’’.7. This result was confirmed in 1919 when the apparent angular shift of stars close to the limb of the Sun was measured during a total solar eclipse.
   
2. In a cathode-ray tube, the angle through which the beam is bent, vertically, horizontally, or diagonally.

See also: → deflection; → angle.

The bending of a light ray under the gravitational effect of a massive body. → deflection angle.

See also: → deflection; → light.

To cause to deviate from accurate focus.

See also: → vâ- + → focus.

1. To undergo → deformation.
   

2. To change the form or shape of. → deformable, → deformed, → deformation.

Etymology (EN): From O.Fr. déformer, from L. deformare “to disfigure,” from → de- + → form.

Etymology (PE): Vâdisidan, vâdisândan infinitive from vâdis, from vâ-, → de-, + dis, → form.

Capable of being → deformed. → deformable mirror

See also: → deform + → -able.

A very thin mirror whose shape can be changed by the force applied by many small pistons behind the mirror. Such a mirror is used in the → adaptive optics technique to correct the → wavefront affected by the → atmospheric turbulence. See also → tip-tilt mirror.

See also: → deformable; → mirror.

Altering in the size or shape of a body. See also → deformable.

See also: Verbal noun of → deform.

Having the → form → changed.

See also: Past participle of → deform.

1. No longer in effect or use; not operating or functioning.
   

2. No longer in existence; dead; extinct.

Etymology (EN): M.E., from O.Fr. defunct or directly from L defunctus “dead,” literally “off-duty,” from p.p. of defungi “to discharge, finish,” from → de- “off, completely” + fungi “perform or discharge duty,” from PIE root *bheug- “to enjoy” (cognates: Latin fructus).

Etymology (PE): Nâkâr, “disabled” (Dehxodâ), leterally “not working,” from negation prefix nâ-, → un-, + kâr, kardan,
→ work

A comet that has lost its ability to emit dust and gas and no longer displays the classic cometary features of a nebulous coma and a tail, or a destroyed comet. Comet Pigott The asteroid (944) Hidalgo is considered by some to be a defunct comet because of its unusual orbit. Another case is the asteroid Phaethon.

See also: → defunct; → comet.

The process of producing a quantifiable result in a → fuzzy logic system, given → fuzzy sets and corresponding → membership functions.
Defuzzification is the last step in a fuzzy logic system. After → fuzzy inferencing,
the overall result is a → fuzzy value. This result should be defuzzified to obtain a final → crisp value.

See also: Verbal noun of → defuzzify; → -tion.

To transform a → fuzzy set to a → crisp set in a → fuzzy logic system.

See also: → de-; → fuzzy; → -fy.

1. General: Degenerate state or character. Reverting to an earlier, simpler, state.
   

2. Math.: Limiting case in which a class of object changes its nature so as to belong to another, usually simpler, class.
   

3. Physics: In quatum mechanics, the condition in which two or more different → wave functions of a system correspond to the same energy. In the case of atomic energy levels, the degeneracy can be removed, for example, by applying an electric field to the atom.
   

State or charcater of → degenerate matter.

Etymology (EN): From degener(ate), → degenerate, + -acy suffix of nouns of quality and state.

Etymology (PE): Vâgeni, from vâgen, → degenerate, + noun suffix -i.

Pressure in a degenerate electron or neutron gas. → degenerate matter.

See also: → degeneracy; → pressure.

Characterized by or associated with → degeneracy.

Etymology (EN): L. degeneratus, p.p. of degenerare “depart from one’s kind, fall from ancestral quality,” from → de- + gener-, stem of
genus “race, stock, kind,” gignere “to beget,” cf. with Gk. genos “race, kind,” gonos “birth, offspring,” from PIE base *gen-/*gon-/*gn- “to produce, beget, be born,” cf. Av. zan- “to bear, give birth to a child, be born,” infinitive zazāite, zāta- “born,” zana- “race” (in sruuô.zana- “belonging to the race of the horned ones”), O.Pers. zana- “tribe” (in paru-zana- “consisting of many tribes”),
Skt. janati “begets, bears,” jana- “creature, human being, race, tribe, people.”

Etymology (PE): Vâgen, from vâ-, → de-, + gen “kind,” (as in hamgen “of the same kind, like each other; friend, partner,” from ham- “together,” → com- + gen “kind,” O.Pers./Av. zana- “race; tribe,” cognate with L. genus, as above). Alternatively, gen may be a variant of Mid./Mod.Pers. gôn/gun “kind, type; manner; color, skin color,” from Av. gaona- “hair, hair color, color.”

Same as → white dwarf.

See also: → degenerate; → dwarf.

Highly compressed matter in which the normal atomic structure has broken down and which, because of quantum-mechanical effects, exerts a pressure that is independent of temperature. Bodies with masses less than → Chandrasekhar’s limit (1.4 solar masses) are supported by electron → degeneracy pressure and have densities of about 10⁶ kg/m³. In collapsed stars of mass above 1.4 solar masses, gravity will overwhelm electron degeneracy and further collapse ensues. Electrons combine with protons to form neutrons, so producing a → neutron star. Because neutrons, like electrons, are
→ fermions and therefore subject to the → Pauli exclusion principle, at high enough densities, about 10¹⁴ kg/m³, neutron degeneracy pressure prevents further collapse of the star. For masses larger than 2-3 solar masses, even neutron degeneracy cannot prevent further collapse, and a → black hole is formed.

See also: → degenerate; → matter.

1. To lower in dignity or estimation; bring into contempt.
   

2. To lower in character or quality; debase.
   

3. Chemistry: Of a compound, especially an organic hydrocarbon, to break down (Dictionary.com).

See also: → de-; → grade.

1. Subdivision of an interval in a scale of measurement.
   

2. Geometry: Measure of angle, the 360th part of a circle.
   

3. Math.: Rank of an equation or expression as determined by the sum of the exponents of the variables.

Etymology (EN): From O.Fr. degré, from V.L. *degradus “a step,” from L.L. degredare, from L. → de- “down” + gradus “step.”

Etymology (PE): Darajé, from Ar. darajat “step, ladder.”

The extent of → coherence of an → electromagnetic wave, as indicated by a → dimensionless number.

Since interference takes place when waves are → coherent, using a → Young’s experiment, the degree of coherence
is measured from the → fringe  → visibility, V. It is defined as the ratio V = (I_max - I_min) / (I_max + I_min), where I_max is the intensity at a maximum of the → interference pattern, and I_min is the intensity at a minimum of the interference pattern.

The electromagnetic wave is considered to be highly coherent when the degree of coherence is about 1, incoherent for nearly zero values, and partially coherent for values between 0 and 1.

See also: → degree; → coherence.

Of a → mechanical system, the number of → independent variables needed to describe its configuration.

See also: → degree; → freedom.

The number of electrons a neutral atom has lost in an ionizing physical process (radiation, shock, collision). In spectroscopy, the degree of ionization is indicated by a Roman numeral following the symbol for the element. A neutral atom is indicated by the Roman numeral I, a singly ionized atom, one which has lost one electron, is indicated by II, and so on. Thus O VI indicates an oxygen atom which has lost five electrons.

See also: → degree; → ionization.

The ratio of the intensity of polarized portion of light to the total intensity at a point in the beam.

See also: → degree; → polarization.

Statics: The → energy that must be expended to permanently disturb a specific state of → equilibrium of a body.

See also: → degree; → stability.

The → number of → edges incident on the → vertex.

See also: → degree; → vertex.

The act or process or an instance of dehumanizing.

See also: Verbal noun of → dehumanize.

To deprive of → human qualities or personality.

See also: → de-; → dehumanize.

The smaller and outermost of Mars’ two satellites. It measures 12.6 km in size, and orbits Mars every 30.2 hours at a distance of about 23,500 km. It was discovered by the American astronomer Asaph Hall in 1877.

See also: In Gk. mythology, Deimos, meaning “fear, terror,” is one of the sons of Ares (Mars) and Aphrodite (Venus).

Chemistry: A process in which all charged species are removed from a solution.
The return of an ionized gas to its non-ionized state after the ionizing source has been removed.

See also: From → de- + → ionization.

1. Belief in the existence of a → God on the evidence of → reason and → nature only, with rejection of supernatural revelation (distinguished from → theism).
   

2. Belief in a God who created the world but has since remained indifferent to it (Dictionary.com).

Etymology (EN): From Fr. déisme, from L. de(us) “god,” → deity,

• → -ism.

Etymology (PE): Dâdâr-bâvari, from dâdâr “creator,” → author,

• bâvari, → ism; izad-bâvari, from izad “god,” → deity.

A person who believes in → deism.

See also: → deism; → -ist

1. A god or goddess.
   

2. Divine character or nature, especially that of the Supreme Being; divinity (Dictionary.com).

Etymology (EN): M.E. deite, from O.Fr., from L.L. deitat- (nominative deitas), from L. dei- (combining form of deus “god”)

• -tat- “-ty,” formed after L. divinitas “divinity.”

Etymology (PE): Izad “god;” from Mid.Pers. yazêt “god; angel,” izišn “performance of the religious rites, worship,” yašt “worship, religious ceremony,” yaštan “to venerate, to perform the religious ceremony;” O.Pers. yad- “to worship;” Av. yaz- “to sacrifice, worship, venerate,” yazata- “deities,” yasna- “religious rite” (Mod.Pers. jašn “feast”); Proto-Ir. *iaz- “to sacrifice, worship, venerate.”

In → vector calculus, a vector → partial derivative represented by the symbol → nabla and defined in three dimensions to be:
∇ = (∂/∂x)i + (∂/∂y)j

• (∂/∂z)k.
  It appears in the operations → gradient: ∇f, → divergence: ∇ . E, → curl: ∇ x E, and → Laplacian:
  ∇ . ∇f = ∇²f.

See also: From Gk. alphabet letter delta.

The amount of time by which an event is retarded.

Etymology (EN): From O.Fr. délaier, from dé-→ de- “away, from” + laiier, from laiss(i)er “to leave,” from L. laxare “to loosen, release, set free.”

Etymology (PE): Derang, from Mid.Pers. dirang, drang “delay, lateness; long, lasting,” Av. drənj- “to fix, fasten, hold,” Proto-Iranian *dra(n)j- “to fix, fasten, hold.”

Same as → delay.

See also: → delay; → time.

Neutrons resulting from nuclear fission which are emitted with a measurable time delay. Delayed neutrons are responsible for the ability to control the rate at which power can rise in a reactor. → prompt neutrons.

See also: Delayed, p.p. of the verbal form of → delay; → neutron.

A mechanism predicted by theoretical models of → supernova explosion that operates after the → supernova shock fails to deliver a → prompt supernova explosion. The delayed supernova explosion mechanism assumes that a few tenth of a second after the → iron core collapse, the supernova shock is stalled due to energy dissipation.
The material between the → protoneutron star and the stalled shock is mainly disintegrated into neutrons and protons due to the high temperatures (a few MeV) in this region. As the → neutrinos coming from the protoneutron star run through this material, a fraction of the neutrinos
are captured by the → nucleons, and their energy is deposited in the material. As a result,
the material behind the shock is heated by the neutrinos. If this neutrino heating is efficient enough, the stalled shock can be reinvigorated to bring about a supernova explosion.

See also: → delay; → supernova; → explosion.

To strike out or remove (something written or printed); cancel; erase.

Etymology (EN): L. deletus, p.p. of delere “destroy, blot out, efface,” from delevi, fro delinere “to erase by smudging,” from → de- “from, away”

• linere “to smear, wipe.”

Etymology (PE): Rušidan, from Kurd. rušê “to be wiped off through rubbing;” cf. Av. fra.uruxti- “destruction;” Wakhi riz-, rəz-/r&#601zd- “to tear apart, rip up a seam;” Yaghnobi ruc/ructa, rušta “to shave off the skin, skin off;” Proto-Ir. *rauj- “to break, bust” (Cheung 2007).

1. An act or instance of deleting. The state of being deleted. A deleted word, passage, etc.
   

2. Genetics: A type of chromosomal aberration in which a segment of the chromosome is removed or lost (Dictionary.com).

See also: Verbal noun from → delete.

1. Fine in texture, quality, construction, etc.
   

2. Fragile; easily damaged; frail (Dictionary.com).

Etymology (EN): M.E. delicat, from L. delicatus “alluring, delightful, dainty,” of uncertain origin.

Etymology (PE): Dârmé, from Mid.Pers. dârmag “delicate;” cf. (dialect of Ferdows) dermi “fine thread,” Sogd. žâm, &#382am “delicate.”

The Dolphin. A small northern constellation, lying just north of the celestial equator between → Pegasus and → Aquila. Abbreviation: Del;genitive: Delphini.

Etymology (EN): L. delphinus, from Gk. delphin-, delphis; akin to Gk. delphys womb, cf. Skt. garbha- “womb; interior.”

Etymology (PE): Dolfin loanword from Gk.

1. The fourth letter of the Greek alphabet (δ, Δ).
   

2. Anything triangular, like the Greek capital delta (δ).
   

3. Math.: An incremental change in a variable.

See also: M.E. deltha, from L. delta, from Gk. delta; from the Phoenician name of the corresponding letter daleth “tent door.”

The prototype of classical → Cepheid variables, which is a pulsating → yellow supergiant. John Goodricke was the first in 1784 to discover its variability. The star shows a quick and sharp rise from minimum to maximum, and slowly declines to its minimum again. The changes in brightness are accompanied by and principally caused by changes in stellar temperature and also by changes in radius. δ Cephei was actually the second Cepheid variable to be discovered. The first one, Eta Aquilae, had been discovered earlier the same year by Edward Pigott. δ Cephei varies with a period of 5.366341 days (or 5 days 8 hours 37.5 minutes) from magnitude 3.48, spectral type F5 Ib in its maximum to magnitude 4.37, spectral type G2 Ib in its minimum. It lies at a distance of 1,340 → light-years.

See also: → delta; Cephei, genitive of → Cepheus.

The → mechanism that imparts or transfers power to a → telescope so that it can move along the → declination direction. See also → tracking.

See also: → delta; → drive.

Same as → Dirac function.

A short distance from the target, in → declination, where the → telescope is pointed for various purposes.

See also: → delta; → offset.

Same as → Mintaka.

See also: → delta; → Orion.

A → binary star in the constellation → Scorpius. Its other designations include BD-22 4068, HD 143275, HR 5953, IRAS 15573-2228, SAO 184014. The → primary star is called → Dschubba. δ Scorpii is one of the brightest stars in the sky. Until 2000, its → visual magnitude was V = 2.32; since then, and due to its transition to a Be phase (→ Be star), it has been even brighter (V = 1.6 mag). It was resolved interferometrically into two components in the 1970s, and the observations indicated a very → eccentric orbit (e ~ 0.94) with a period of ~10.6 years. The → binary system is not → eclipsing, and the → secondary star is 1.78 ± 0.03 mag fainter than the primary one. The → spectral type of the primary is B0.5 V and that of the secondary B2V. The components are therefore of similar size and thus may produce strong interactions between themselves and affect the → circumstellar disk at, or near → periastron. δ Sco was first classified as a Be star when a small amount of Hα emission was observed in its spectrum. Since the reclassification of δ Sco as a Be star, two periastrons have passed, once in 2000, and again in 2011. Spectroscopic observations around the 2000 periastron revealed a large increase in the Hα emission compared to that found previously in 1993, with further noticeable month-to-month variations in its Hα → equivalent width and visual magnitude. It has been suggested that these small variations are due to the disk’s inability to grow greater than the → Roche lobe of the primary, which caused a density increase on the side of the disk facing the secondary (See Miroshnichenko et al., 2013, AJ 766, 119 and references therein).

See also: Delta Scorpii is the system’s → Bayer designation.

A member of a class of → pulsating stars with periods less than 0.3 days, → spectral types A or F, and visual light amplitudes in the range from a few thousands of a magnitude to about 0.8 mag. On the → H-R diagram,
δ Scuti stars form a group which lies in an → instability strip which includes the classical → Cepheids at its brightest end and the pulsating → white dwarfs at its faintest limit. These stars can show very complex light variations since, while some of them are pulsating in one radial mode only, others may be pulsating simultaneously in several radial and non-radial modes.

See also: Named after the prototype star δ of constellation → Scutum; → variable.

A measure of the variation in → Earth’s rotation, which is the difference between → Terrestrial Time (TT) and → Universal Time (UT). TT is uniform and related to the → International Atomic Time, whereas UT, which is directly tied to the Earth’s rotation, is not strictly uniform and shows small erratic fluctuations. Between 1970 and 1990, ΔT changed from +40 to +57 seconds, and was +67 seconds for 2010.

See also: Δ, Gk. letter of alphabet indicating a difference; T for → time.

In communications, the process of recovering the original information from a modulated signal wave. → modulation.

See also: → de- + → modulation

An imaginary intelligent entity introduced in several → thought experiments, such as → Laplace’s demon and → Maxwell’s demon.

Etymology (EN): From L. dæmon “spirit,” from Gk. daimon “deity, fate, fortune.”

Etymology (PE): Pari “a good genius, a fairy,” from Mid.Pers. parîk “sorceress, witch;” from Av. pairikā- “sorceress, witch.”

1. To make evident or establish by arguments or reasoning; prove.
   

2. To describe, explain, or illustrate by examples, specimens, experiments, or the like.
   

3. To manifest or exhibit; show.
   

4. T display openly or publicly, as feelings (Dictionary.com).

Etymology (EN): From L. demonstratus, p.p. of demonstrare “to show, point out,” from
→ de- + monstrare “to show,” from monstrum “sign, portent.”

Etymology (PE): Padišidan, from Sogd. padēš “to show,” ultimately from Proto-Ir. *apa-dais-, from *dais- “to show,” cf. Av. daēs- “to show,” related to andiš, → think.

1. The → act or → circumstance of proving or being → proved conclusively, as by → reasoning or a show of → evidence.
   

2. Something serving as → proof or supporting evidence.
   

3. An exhibition, as of feeling; display; → manifestation (Dictionary.com).

See also: Verbal noun of → demonstrate.

A person or thing that demonstrates (Dictionary.com).

See also: → demonstrate; → -or.

The removal of mystery or confusion surrounding a topic or idea.

See also: → demystify; → -tion.

To rid of mystery or obscurity; clarify.

The removal of mystery or confusion surrounding a topic or idea.

See also: → de-; → mystify;

1. A branching treelike figure produced on or in a mineral by a foreign mineral.
   

   2. A crystallized → arborescent form.
      

   3. Any of the usually branching protoplasmic processes that conduct impulses toward the body of a neuron (Merriam-Webster.com).

Etymology (EN): From Gk. dendrites “of or pertaining to a tree,” from dendron “tree,” cognate with Pers. deraxt, variant dâr, → tree.

Etymology (PE): Šâxzâré, from šâxzâr, from šâxsâr
“full of branches,” from šâx, šâxé, → branch, + suffix -zâr denoting profusion, and abundance, as in kârzâr “a field of battle; combat” šurezâr “unfertile, salty ground; nitrous earth,” xoškzâr “arid land,” and so forth.

The branch of → botany dealing with trees and shrubs. Dendrology studies the distinguishing characteristics of tree species for the purpose of identification and classification into orders and other natural groups.

See also: From dendro- a combining form meaning “tree,” from Gk. dendron “tree,” related to Pers. dâr, → tree, + → -logy.

The brightest star, of magnitude V = 1.25, in the constellation → Cygnus. It is a → supergiant of → spectral type A2 Ia.

Etymology (EN): Deneb “tail,” from Ar. dhanab ad-dajajah (ذنب الدجاجه) “tail of the hen,” referring to the legendary swan (Cygnus) in Gk. mythology.

Etymology (PE): Zanab, from Ar., as above.

The second brightest star, of magnitude V = 2.14, in the constellation → Leo. A → main sequence star of → spectral type A3 V.

Etymology (EN): Denebola, from Ar. dhanab al-asad (ذنب الاسد) “tail of the lion,” referring to the lion in Gk. mythology.

Etymology (PE): Širdom “tail of the lion,” from šir “lion” → Leo

• dom “tail,” → comet. Zanab-ol-asad from Ar., as above.

1. An assertion that something said, believed, alleged, etc., is false.
   

2. Refusal to believe a doctrine, theory, or the like.
   

3. Disbelief in the existence or reality of a thing (Dictionary.com).

See also: → deny; → -al.

1. To give a specific name to.
   

   2. To express in a multiple of a unit of measurement.

See also: → de-; → nominate.

1. A name or designation.
   

2. The action of naming or classifying something.

3. One of the grades or degrees in a series of designations of quantity, value, measure, weight, etc. (Dictionary.com).

See also: Verbal noun of → denominate.

The quantity y in a fraction x/y. The quantity x is
the → numerator.

See also: → denominate; → -or.

1. To condemn or censure openly or publicly.
   

2. To make a formal accusation against, as to the police or in a court (Dictionary.com).

See also: → de-; → announce.

Having relatively high → density.
Math.: The quality of a subset A of a topological space X, indicating that
any point in X can be well approximated by points in A.

Etymology (EN): From L. densus “thick, crowded,” cognate with Gk. dasys “hairy, bushy, thick grown.”

Etymology (PE): Cagâl “dense, thick,” related to ceqer “stiff, hard, tough, firm” (dialectal Kermâni ceqel, Šândizi caqal),
caqâlé “stiff, unripe fruit.”

An opaque region of a → molecular cloud (A_V  10 mag) which is considered to be the progenitor of → star formation.
Dense cores have temperatures of about 10 K and masses of roughly 1 to 10Msun each and in which the → molecular hydrogen density is roughly 10⁴-10⁵ cm^-3 and size 0.1 pc. The → self-gravity of a dense core plays a central part in star formation. See also → hot molecular core.

See also: → dense; → core.

→ core mass function.

See also: → dense; → core; → mass; → function.

A type of → interstellar medium cloud in which → carbon (C) becomes almost completely molecular due to relatively high → extinction. The chemistry is qualitatively different from that of → diffuse molecular clouds, as the → electron abundance is very low (→ cosmic-ray ionization being the dominant source) and the reactive C is replaced by the very stable → carbon monoxide (CO). This regime is found only in → sightlines with A_V > 5-10 mag; not all such sightlines will contain dense cloud material and if dense cloud material is present it is likely to be surrounded by → translucent material.

These clouds are typically → <i><a class="linkVoir" href="/terms/self-gravitating/">self-gravitating</a></i>, 
and are most often observed by → <i><a class="linkVoir" href="/terms/infrared/">infrared</a></i> absorption and
→ <i><a class="linkVoir" href="/terms/millimeter-wave/">millimeter wave</a></i> emission methods. Their
 densities are typically at least 10<SUP>4</SUP> cm<SUP>-3</SUP>, and their
 → <i><a class="linkVoir" href="/terms/kinetic-temperature/">kinetic temperature</a></i>s are typically on the order of 10-50 K in
 the quiescent regions. Most of the more than 140 currently known
 → <i><a class="linkVoir" href="/terms/interstellar-molecule/">interstellar molecule</a></i>s 
 were found through observations of → <i><a class="linkVoir" href="/terms/microwave/">microwave</a></i>→ <i><a class="linkVoir" href="/terms/rotational-transition/">rotational transition</a></i>s 
in such clouds, starting with the discovery of OH, followed by a host of other new
 detections such as CO, NH<SUB>3</SUB>, H<SUB>2</SUB>O, and H<SUB>2</SUB>CO
  (Snow & McCall, 2006, ARA&A 44, 367).

See also: → dense; → molecular; → cloud.

The amount of any quantity per unit volume. The mass density is the mass per unit volume. The energy density is the energy per unit volume; particle density is the number of particles per unit volume.
See also:
→ charge density, → column density, → critical density, → current density, → density fluctuation, → density parameter, → density profile, → density wave, → density-bounded H II region, → density-wave theory, → electron density, → energy density, → flux density, → magnetic flux density, → maximum density of water, → neutral density filter, → nuclear density, → number density, → optical density, → period-mean density relation, → Planck density, → potential density, → power spectral density, → probability density function, → radio flux density, → relative density, → specific density, → spectral density, → surface density.

See also: Noun form of → dense.

A localized increase in number of → stellar black holes near a → supermassive black hole predicted by models of galactic → stellar dynamics (Bahcall, Wolf, 1976, ApJ, 209, 214). Same as → stellar cusp.

See also: → density; → cusp.

In the early Universe, localized enhancements in the density of either matter alone or matter and radiation. According to models, very small initial fluctuations (less than 1 percent) can lead to subsequent formation of galaxies.

See also: → density; → fluctuation.

The number of units of mass of the → chemical element that are present in a certain volume of a medium. The density of an element depends on temperature and pressure. The element Osmium has the highest known density: 22.61 g/cc; in comparison gold is 19.32 g/cc and lead 11.35 g/cc.

See also: → density; → element.

One of the four terms that describe an arranged version of the → Friedmann equations. They are all time dependent.

1. For matter:
   Ω_m = 8πGρ_m/(3H²), where G is the → gravitational constant, ρ_m is the mean matter density, and H the → Hubble parameter. The matter density parameter is also expressed as Ω_m = ρ_m/ρ_crit, where ρ_crit is the → critical density.
   

2. For radiation: Ω_r = 8πGρ_r/(3H²), where ρ_r is the radiation equivalent of matter density.
   This parameter is also expressed as Ω_r = ρ_r/ρ_crit.
   

3. For the → cosmological constant: Ω_Λ = Λc²/(3H²). Similarly, Ω_Λ = ρ_Λ/ρ_crit, where &rho_Λ = Λc²/(8πG)
   is sometimes referred to as the density of → dark energy.
   

4. For the → curvature of space-timeΩ_k = -kc²/(R²H²), where k is the → curvature constant and R the → cosmic scale factor.
   

Note that: Ω_m + Ω_r + Ω_Λ + Ω_k = 1, and Ω_total = Ω_m + Ω_r + Ω_Λ = 1 - Ω_k.

See also: → density; → parameter.

1. A → profile representing the → density of a quantity.
   

2. A → profile representing the distribution of stars as a function of their number in a region.

See also: → density; → profile.

A wave phenomenon in which the density fluctuations of a physical quantity propagates in a compressible medium. For example, the → spiral arms of a → galaxy are believed to be due to a density wave which results from the natural instability of the → galactic disk caused by its own gravitational force. A common example of a density wave concerns traffic flow. A slow-moving vehicle on a narrow two-lane road causes a high density of cars to pile up behind it. As it moves down the highway the “traffic density wave” moves slowly too. But the density wave of cars does not keep the same cars in it. Instead, the first cars leave the density wave when they pass the slow vehicle and continue on at a more normal speed and new ones are added as they approach the density wave from behind. Moreover, the speed with which the density wave moves is lower than the average speed of the traffic and that the density wave can persist well after its original cause is gone. See → density wave theory.

See also: → density; → wave.

One possible explanation for → spiral arms,
first put forward by B. Lindblad in about 1925 and developed later by C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material structures, but regions of somewhat enhanced density, created by → density waves. Density waves are perturbations amplified by the self-gravity of the → galactic disk. The perturbation results from natural
non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters. Density waves rotate around the → galactic center and periodically compress the disk material upon their passage. If the spiral arms were rigid structures rotating like a pinwheel,
the → differential rotation of the galaxy would wind up the arms completely in a relatively short time (with respect to the age of the galaxy), → winding problem.
Inside the region defined by the → corotation radius,
density waves rotate more slowly than the galaxy’s stars and gas; outside that region they rotate faster.

As the density waves rotate, they are overtaken by the individual stars and nebulae/molecular clouds that are rotating around the galaxy at a higher rate. The molecular clouds passing through the density wave are subjected to compression because it is a region of higher density. This triggers the formation of clusters of new stars, which continue to move through the density wave.

The short-lived stars die, most likely as supernovae, before they can leave the spiral density wave. But the longer-lived stars that are formed pass through the density wave and eventually emerge on its front side and continue on their way as a slowly dissipating cluster of stars.

Density wave theory explains much of the spiral structure that we see, but there are some problems. First, computer simulations with density waves tend to produce very orderly “grand design” spirals with a well-defined, wrapped 2-arm structure. But there are many spiral galaxies that have a more complex structure than this (→ flocculent spiral galaxy). Second, density wave theory assumes the existence of spiral density waves and then explores the consequences.
See also:
→ stochastic self-propagating star formation.

See also: → density; → wave; → theory.

An → H II region which lacks enough matter to absorb all → Lyman continuum photons of the → exciting star(s). In such an H II region a part of the ionizing photons escape into the → interstellar medium. See also → ionization-bounded H II region.

See also: → density; → bounded; → region.

To state that (something declared or believed to be true) is not true (Dictionary.com).

Etymology (EN): M.E. denien, from O.Fr. denoiir “deny, repudiate, withhold,” from L. denegare “to deny, reject, refuse,” from → de- “away” + negare “refuse, say no,”
from Old L. nec “not,” from PIE root *ne- “no, not.”

Etymology (PE): Niyârdan, literally “to bring (say) no,”
from ni-, variant of na→ not, → non-, → un-, + ârdan contraction of âvardan “to bring; to cause or produce,” → production.

1. General: Determined or conditioned by something else.
   

2. Math.: A variable whose value depends on the value assigned to another variable.
   

3. Statistics: An event which is not independent.

Etymology (EN): M.E. dependant, from M.Fr., pr.p. of dépendre, from L. dependere, from → de- + pendere “to hang, be suspended,” PIE base *(s)pen(d)- “to pull, stretch.”

Etymology (PE): Vâbasté, from vâ-→ de- + basté p.p. of bastan “to bind, shut,” from Mid.Pers. bastan/vastan “to bind, shut,” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” cf.
Skt. bandh- “to bind, tie, fasten,” PIE *bhendh- “to bind” (Ger. binden, E. bind).

Math.: A variable whose value depends on the value assigned to another value. For example, in the equation y = 2x, the value of y depends on that of x. See also → independent variable.

See also: → dependent; → variable.

To make two signals out of phase. For example, to get one signal at its highest peak while the other signal is at its lowest peak; they will be 180 degrees out of phase.

See also: → de-; → phase.

Same as → out of phase.

See also: Past participle of → dephase.

To decrease markedly the supply or abundance of; exhaust; to cause → depletion.

Etymology (EN): From L.L. depletionem “blood-letting,” from L. deplere “to un-fill, to empty out,” from → de- “off, away” + plere “to fill,” from plenus “full,” from PIE *ple- “to be full,” PIE base *pelu- “full,”
cf. O.Pers. paru- “much, many,” Av. parav-, pauru-, pouru-, par- “to fill,”
Mod.Pers. por “full,” Skt. puru-, Gk. polus. por “full,” Gk. polus, O.E. full “full,” from P.Gmc. *fullaz, O.H.G. fol, Ger. voll, Goth. full.

Etymology (PE): Tisâyidan, from Tabari tisâ “empty, naked, bare” + -idan infinitive suffix. Variants of tisâ in dialects and literary Pers.: Saraxsi, Lâsgardi, Sangesari tusâ “empty, naked,” Aftari tussâ “empty,” literary Pers.
tohi “empty,” Mid.Pers. tuhig, Av. taoš- “to become empty,” pres. tusa-, caus. taošaya-, tusən “they lose their posture,” Skt. tuccha-, tucchya- “empty,” L. tesqua, tesca “deserted place,” Russian tošcij “hollow;” PIE base *teus- “to empty.”

General: The act or process of depleting. The state of being depleted.
Astro.: The process or state of a material which contains less of a particular chemical element or isotope than the expected standard amount.

See also: Noun from → deplete

The region of a semiconductor in which the density of mobile carriers is too low to neutralize the fixed charge density of donors and acceptors.

See also: → depletion; → layer.

The time it takes for a given → chemical species to be reduced below a significant level of → abundance in a → compound or → object.

See also: → depletion; → time.

Reducing or randomizing the polarization of an electromagnetic wave, by transmission through a non-homogeneous medium or a depolarizer.

See also: from → de- + → polarization.

To remove or reduce the population of, as by destruction or expulsion (Dictionary.com).

See also: → de-; → populate.

To expel forcibly a foreigner from a country; banish.

See also: → de-; → export.

The action of deporting an alien from a country.

See also: → deport; → -tion.

1a) Something precipitated, delivered and left, thrown down, or accumulated, as by a natural process.

1b) Substance which settles down from a solution or a suspension, such as the natural sediment of wine in a bottle. See also → sediment.

2. To leave or form a layer of some substance (sand, sediment, etc.) as a gradual process in one place, or be left in this way.

Etymology (EN): From L. depositus, p.p. of deponere “to lay aside, put down,” from → de- “away” + ponere “to put,” → position.

Etymology (PE): Lerd “sediment, tartar of wine,” probably a variant of dord “dregs, lees, sediment, tartar of wine.”

The process by which water vapor changes directly to ice without first becoming a liquid. This is how snow forms in clouds, as well as frost and hoar frost on the ground. The opposite of deposition is → sublimation.
→ condensation.

Etymology (EN): M.E., from O.Fr. deposition, from L. deposition- “putting aside, testimony, burial,” from deposit(us) “laid down,” p.p. of deponere “to put down,” from → de- +
ponere “to put, place.”

Etymology (PE): Vâneheš, from vâ-→ de- + neheš verbal noun of nehâdan “to put, place,” Mid.Pers. nihâtan, O.Pers./Av. ni- “down; into,” → ni-, + dā- “to put; to establish; to give,” dadāiti “he gives,” cf. Skt. dadâti “he gives,” Gk. didomi “I give,” L. do “I give;” PIE base *do- “to give.”

An act of attacking or plundering.

See also: → de-; → predation.

1. Navigation:→ dip.
   
2. Meteorology: An area of low atmospheric pressure.
   
3. Geology: An area of the Earth’s crust sunken or depressed below the surrounding area, formed by various mechanisms.

Etymology (EN): Depression, verbal noun of depress, from O.Fr. depresser, from L.L. depressare, frequentative of pressus, p.p. of deprimere “press down,” from → de- “down” + premere “to press.”

Etymology (PE): 1) Našib-e ofoq, from našib “declivity, descent; lowness of ground, slope of any place” (Mid.Pers. nišēp “declivity, (astrology) dejection,” Av. *nixšvaēpā-, xšvaēpā- “bottom, rear”)

• ofoq→ horizon. 2) Forubâr, from foru “down, downward; below; beneath,” → de-
• bâr “load, pressure, charge,” → charge. 3) Našiblâx, from našib, as above, in item 1, + -lâx place suffix.

To remove or withhold something from the enjoyment or possession of (Dictionary.com).

Etymology (EN): M.E. depriven, from O.Fr. depriver, from L. deprivare, from → de- + privare “to separate, rob,” → private

Etymology (PE): Âzvandan, from âz- “out, away,” → ex-,

• van, contraction of vand, band “to bind, fasten, close,” → private.

Noun form of → deep. → optical depth.

Etymology (EN): From M.E. depthe, from O.E. deop “deep,” → deep

• -the “-th.”

Etymology (PE): Žarfâ, noun of žarf “deep,” → deep + -â.

1. General: The act or fact of deriving or being derived; origin; descent.
   

2. Math.:→ differentiation; development of a theorem.
   

3. Grammar: The formation of a word from another word or base. It typically occurs by the addition of an affix.

See also: Verbal noun of → derive.

1. General: Derived from another; not original or primitive.
   

2. Math.: For a → function  y = f(x), the → limit of the → ratio of the → increment of the function, Δy, to the increment of the → variable, Δx, as the latter tends to → zero. Derived function; result of → differentiation.
   

3. Chemistry: Substance derived or prepared from some other → substance.

Etymology (EN): From M.E. derivatif, from L.L. derivativus, from derivat(us), p.p. of derivare, → derive

• -ivus.

Etymology (PE): Vâxané, noun from vâxan, present stem of vâxanidan,
→ derive.

1. General: To conduct, draw, obtain from a source or origin; to trace to an origin.
   

2. Chemistry: To produce or obtain (a → substance) from another.
   

3. Math.: Verbal form of → derivation.

Etymology (EN): From O.Fr. dériver, from L. derivare “to draw off (a stream of water), from its source,” from de- “from” + riv(us) “a stream” + -are infinitive suffix.

Etymology (PE): Vâxanidan, from vâ- prefix which indicates “returning to, reference,”
→ de-, + xan “source,” variant xân (Gilaki xoni, Tabari xoni, Laki kyani, Tâleši xâni, xoni,
Mid.Pers. xân, xânig “source, spring,” Av. xā-, xan- “source, fountain, spring,” xayana- “belonging to a spring,” cf. Khotanese khâhâ- “spring, fountain,” Skt. khâ’- “spring, source”)

• -idan infinitive suffix.

Determined by mathematical computation; formed or developed from something else; not original.

See also: P.p. of → derive.

René Descartes (1596-1650), French mathematician, physicist, and philosopher, who made valuable contributions to mathematics, physics, and philosophy. Descartes was a pioneer and major figure in 17th century rationalism. He represents a major break with the Aristotelianism and Scholasticism of the Medieval period. René Descartes is regarded as the father of modern philosophy. → Descartes ray, → Cartesian coordinates, → Cartesian vortex theory.

Same as → rainbow ray.

See also: → Descartes; → ray.

A formula that gives the position of an image formed by highly → paraxial rays from a → spherical mirror. It is quite accurately given by: 1/x_o + 1/x_i = 2/x_C, where x_o is the distance along the → principal axis from the mirror to the object, x_i is the distance from mirror to image, and x_C is the distance from the mirror to its center of curvature. Any distance measured on the same side of the mirror as the reflecting surface is positive; on the other side, negative. Thus for a → concave mirror  x_C is positive; for a → convex mirror, negative.

See also: → Descartes; → formula.

1. To go or pass from a higher to a lower place.
   

   2. To be passed on by parents or ancestors; be inherited

Etymology (EN): M.E. descenden, “move or pass from a higher to a lower place,” from O.Fr. descendre “descend, dismount; fall into; originate in,” from Latin descendere “come down, descend, sink,” from → de- “down” + scandere “to climb,” from PIE root *skand- “to spring, leap, climb.”

1. A person, animal, or plant that is descended from a specific individual, race, species, etc.
   

   2. Something deriving in appearance, function, or general character from an earlier form.

See also: → descend.

The point in an → orbit where the orbiting body crosses a reference plane, such as the → ecliptic or the → celestial equator, going from north to south. → ascending node.

See also: → descend; → -ing; → node.

1. General: To tell or represent in written or spoken words; give an account of.
   

2. Geometry: To draw or trace the outline of.

Etymology (EN): M.E. describen, from L. describere, from → de-

• scribere “to write,” from PIE *skreibh- (cf. Gk. skariphasthai “to scratch an outline, sketch;” Lett. skripat “scratch, write;” O.N. hrifa “scratch”); PIE base *sker- “cut, incise.”

Etymology (PE): Vâveštan, vâvisidan (on the model of neveštan, nevisidan “to write”), from vâ-, → de-, + vešt-, vis-, → inscribe.

1. The act or method of describing.
   

2. A statement, picture in words, or account that describes.
   

3. Geometry: The act or process of describing a figure.

See also: Verbal noun of → describe.

Serving to describe; containing description.

See also: → describe; → -ive.

1. An arid land where average annual precipitation is so low (usually taken as below 250 mm mean annual rainfall) that it will not adequately support vegetation, and where the potential rate of evaporation far exceeds precipitation.
   

2. Any place lacking in something; → brown dwarf desert.

Etymology (EN): From M.E., from O.Fr. desert, from L.L. desertum, literally “thing abandoned,” from deserere “to abandon.”

Etymology (PE): Kavir “salt and sour ground, where nothing grows; wilderness,” of unknown origin.

1a) An outline, sketch, or plan, as of the form and structure of a work of art, an edifice, or a machine to be executed or constructed.

1b) Organization or structure of formal elements in a work of art; composition.

2a) To prepare the preliminary sketch or the plans for (a work to be executed), especially to plan the form and structure of.

2b) To plan and fashion artistically or skillfully.

2c) To intend for a definite purpose (Dictionary.com).

Etymology (EN): M.E. designen, from M.Fr. desseign “purpose, project, design,” from It. disegno, from disegnare “to mark out,” from L. designare “mark out, devise, designate, appoint,” from → de- “out” + signare “to mark,” from signum “a mark, → sign.”

Etymology (PE): Barsé, related to (Delijâni) barsi “to throw,” variants baysi, vaesi, deresi; (Xonji, Gerâši) bar-, barressa “to fall down;” ultimately from Proto-Ir. *garH- “to throw;” cf. (+*ni-) Av. niγr- “to throw down;” Khotanese bīr- “to throw, sow;” Pers. garzin “a pointed arrow;” Pashto qoer “jump, leap,” aqar “fitting an arrow to the bow-string ready to shoot;” cf. Skt. gar- “to raise a weapon;” Gk. ballein “to throw,” → problem, blema “projectile;” PIE root *g^welH- “to throw” (Cheung 2007). Barsé is coined on the model of Ger. Entwurf “design; project” and entwerfen “to design,” from werfen “to throw;” Pers. dar-andâxtan “to propound, to pose” (Hâfez: falak râ saxt beškâfim o tarhi now dar-andâzim); also Ar. tarh (طرح) “to throw.”

A distinctive name or title; appellation. → Bayer designation; → Flamsteed designation; → variable star designation.

Etymology (EN): From L. designatus, p.p. of designare “to mark out, choose, appoint,” from → de- “out” + signare “to mark,” from signum, → sign.

Etymology (PE): Nâmgozini, from nâm, → name, + gozini “choosing,” from gozidan “to chose,” → select.

A person who devises or executes designs, especially one who creates forms, structures, and patterns, as for works of art or machines
(Dictionary.com).

See also: → design; → -er.

Verbal form of → desorption.

Etymology (EN): Desorb, from → de- + -sorb, modeled on
→ absorb.

Etymology (PE): vâšamidan, from vâ-, → de-, + šamidan, modeled on baršamidan, → absorb.

A physical or chemical process by which a substance that has been sorbed (adsorbed or absorbed) by a liquid or solid material is removed from the material. Reverse of → sorption. → absorption;
→ adsorption.

See also: Verbal noun of → desorb.

In spite of; notwithstanding.

Etymology (EN): M.E. despit, from O.Fr. despit, from L. despectus “a looking down on, scorn, contempt,” from p.p. of despicere “to look down on, scorn,” from → de- “down” + spicere/specere “to look at,” → scope; the preposition is contraction of in despite of, a loan-translation of O.Fr. en despit de “in contempt of.”

Etymology (PE): Pâdkâm, literally “against (one’s) will or desire,” from pâd- “against, contrary,” → anti-, + kâm “desire, wish;” Mid.Pers. kâm “desire, wish;” cf. Skt. kāma- “desire, wish.”

An → interference process in which the
→ amplitude of the the amplitude of the resultant wave is less than that of either individual waves. See also
→ constructive interference.

Etymology (EN): Destructive, from O.Fr. destructif, from L.L. destructivus, from destruct-, p.p. stem of destruere, from → de-

• struere “to build,” → structure; → interference.

Etymology (PE): Andarzaneš, → interference; virângar “destructive,” from virân, “destroyed, ruined.”

A binary system whose components are not in contact and in which no significant mass exchange is occurring.

Etymology (EN): P.p. of detach, from O.Fr. destachier (Fr. détacher), from des- “apart,” + -tachier (as in atachier “to attach”); → system.

Etymology (PE): Râžmân→ system; jodâ “separate, distinct,” Mid.Pers. jutâk (yutâk), jut (yut).

1. An individual or minute part; an item or → particular.
   

2. To relate or report with complete particulars; tell fully and distinctly (Dictionary.com).

Etymology (EN): From Fr. détail, from O.Fr. detail “small piece or quantity,” literally “a cutting in pieces,” from detaillier “to cut in pieces,” from → de- “entirely” + taillier “to cut in pieces,” from L. taliare “to split.”

Etymology (PE): Pâzé, from Sogd. pâzê “portion, bit;” Kurd. pâž “part of a whole;” related to Mod.Pers. baxš “part, division,” bâž “tribute, toll, impost;” ultimately from Proto-Ir. *baj- “to bestow, divide, have a share;” cf. Av. baž- (baxš-) “to divide, bestow, give a share;” O.Pers. bāji- “tribute, tax;” Mid.Pers.
bâj, bâž “tribute, tax,” baxtan “to distribute,” baxt “luck, fate;” Skt. bhaj- “to share, distribute, apportion,” Gk. phagein “to eat (to have a share of food)”; PIE base *bhag- “to share out, apportion;” → division.

1. Having many details.
   

2. Thorough in the treatment of details (Dictionary.com).

See also: Adjective from → detail.

To uncover the presence of, as of a sought-for nuclear particle or an astronomical object.
In radio communication, to extract the signal from its modulated carrier wave.

Etymology (EN): From M.E., from L. detectus, p.p. of detegere “uncover, disclose,” from → de- reversal prefix + tegere “to cover.”

Etymology (PE): škâridan “to reveal,” infinitive of âškâr “manifest, clear,” Mid.Pers. âškârak “manifest,” from Proto-Iranian *âviškâra- from *âviš, Av. âviš “manifestly,” + *kâra-, from kar- “to do, make,” cf. Skt. avistya- “apparent, manifest.”

The act or state of detecting, → detect
Any operation on an electrical or electromagnetic signal to recover information from it.

Etymology (EN): M.E., from L.L. detection-, from detect(us), → detect, + -ion.

Etymology (PE): škâreš, from âškâr present-tense stem of âškâridan→ detect + -š verbal noun suffix.

The square of the ratio of the output → signal-to-noise
(S/N) to the input S/N.

See also: Detective, adj. of → detect; → quantum; → efficiency.

A performance criterion for an electronic detector, reciprocal of the corresponding → noise-equivalent power (D = 1/NEP).

See also: From detective, relating to → detect + → -ity.

A device whose main function is detection.
Any device or array of electronic sensors connected to a computer, used to detect the passage of a particle or photon; e.g. a → charge-coupled device (CCD).

Etymology (EN): L.L. detector, from deteg(ere), → detect,

• -tor suffix forming agent noun.

Etymology (PE): škârgar, from âškâr present-tense stem of âškâridan “to detect” + -gar a suffix of agent nouns, from kar-, kardan “to do, to make” (Mid.Pers. kardan, O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “makes,” cf. Skt. kr- “to do, to make,” krnoti “makes,” karma “act, deed;” PIE base k^wer- “to do, to make”).

An agent or factor that determines the nature of something or that fixes or conditions an outcome.
Math.: An algebraic expression used in the solution of systems of linear equations, which consists of the sum of products of elements, each with an appropriate algebraic sign, usually written in a square array.

Etymology (EN): From → determine + -ant suffix forming noun.

Etymology (PE): Âtarmgar, from âtarm present stem of âtarmidan→ determine + -gar, → detector.

The act of deciding definitely and firmly; the result of such an act of decision.

See also: Verbal noun of → determine.

1. General: To settle or decide by choice of alternatives or possibilities.
   

2. Math.: To fix or define the position, form, or configuration of.
   

3. Logic: To explain or limit by adding differences.

Etymology (EN): From O.Fr. déterminer, from L. determinare “set limits to,” from → de- + terminare “to mark the end or boundary,” from terminus “boundary, border, end,” → term.

Etymology (PE): Âtarmidan, from âtarm + verb forming suffix -idan; âtarm from intensive prefix â- + tarm “limit, boundary,” → term.

1. Decided; settled; resolved.
   

   2. Showing determination, characterized by determination.

See also: p.p. of → determine.

The belief that every event is necessitated by antecedent events and conditions together with the laws of nature. → deterministic physics.

Etymology (EN): From → determine + → -ism.

Etymology (PE): Âtarmbâvari, from âtarm, → determine,

• bâvari, noun of bâvar “beleif;” Mid.Pers. wâbar “beleif;” Proto-Iranian *uar- “to choose; to convince; to believe;” cf. Av. var- “to choose; to convince” varəna-, varana- “conviction, faith;” O.Pers. v(a)r- “to choose; to convince;” Skt. vr- “to choose,” vara- “choosing.”

Of, pertaining to, or dealing with → determinism.

See also: From determinist + → -ic.

The classical representation of the laws of nature according to which a particular future state (B) will arise from a particular past one (A). In contrast to → quantum physics which deals with the probability for the transition from A to B.

See also: Deterministic, adj. of determinism; → physics.

A theory in which specification of the initial value of all relevant variables of the system is sufficient to calculate the past values and
to predict the future values of such variables for any arbitrary
time. Moreover, it is possible, for any arbitrary time, to assign a value to all the variables characterizing the system. In quantum mechanics, the time evolution of the → wave function, governed by the → Schrodinger equation, is deterministic. Quantum mechanics, however, is a non deterministic theory because of the probabilistic nature of the predictions for the values of the → observables of a quantum system.

See also: → deterministic; → theory.

1. To set off a → detonation.
2. To explode or cause to explode.

Etymology (EN): From L. detonatus, p.p. of detonare “to thunder down, roar out,” from → de- + tonare “to thunder,” cf. Pers. tondar “thunder,” Skt. stanáyati “thunders,” tanyatá- “thundering,” Gk. stonos “groan,” stenein “to groan,” Thôrr “the Old Norse god of thunder,”
P.Gmc. thunraz (Du. donder, Ger. Donner “thunder,” E. thunder, Fr. tonnerre), PIE base (s)tene- “to resound, thunder.”

Etymology (PE): Tarâkidan “to split, cleave; to make a noise in splitting,”
variants tarakidan, taraqidan, taraqqé “firecracker,” from tarâk/tarak “split, cleft, crack; the noise of anything when splitting or cleaving,” maybe related to Pers. dar-, daridan “to tear, cut,” Av. dar- “to tear,” dərəta- “cut,” auua.dərənant- “shattering,” Skt. dar- “to crack, split, break, burst,” darati “he splits,” cf. Gk. derma “skin,” E. tear, Ger. zerren “to pull, to tear,” zehren “to undermine, to wear out,” PIE base *der- " to split, peel, flay."

Instantaneous combustion or conversion of a solid, liquid, or gas into larger quantities of expanding gases accompanied by heat, shock, and a noise. → deflagration; → explosion.

See also: Verbal noun of → detonate.

To add → deuterium to a → chemical compound.

Etymology (EN): From L. deuter(ium), → deuterium, + -ate a suffix forming verbs from L. -atus (masc.), -ata (fem.), -atum (neut.).

Etymology (PE): Doteridan, infinitive from doteriom, → deuterium.

Describing a → chemical compound to which → deuterium is added.

See also: Past participle of → deuterate.

A chemical species in which the → deuterium abundance is → enriched with respect to a mean standard value.

See also: → deuterated; → species.

The process of introducing → deuterium into a → chemical compound.

See also: Verbal noun of → deuterate.

The first heavy → isotope of → hydrogen (²H), the → nucleus of which consists of one → proton and one → neutron. Like hydrogen, the deuterium atom has one electron, and therefore has similar chemical properties to hydrogen, forming, e.g., → heavy water (HDO). Deuterium is generated only during → Big Bang nucleosynthesis. It is destroyed in stars through the reaction D + p → ³He + γ (→ deuterium burning). As there is no net source of deuterium in stars, its abundance has decreased steadily since the → Big Bang, and any value measured today must be a lower limit on the primordial value. However, → fractionation processes lead to local → deuterium enhancements; see → deuterium abundance for more details. Theoretical models of Big Bang nucleosynthesis predict D/H to be (2.61 ± 0.15) x 10^-5 (Steigman et al. 2007, MNRAS 378, 576) and this is closely matched by measurements from
intergalactic Dα line absorption observations toward high-redshift quasars
that give 2.53±0.04 x 10^-5 (Cooke et al. 2014, ApJ 781, 31).
See also:

→ deuterated, → deuterated species, → deuterium enrichment, → deuterium enrichment factor, → deuterium fractionation, → deuteron.

See also: From Gk. deutero-, combining form of deuterios “second” + -ium suffix occurring in scientific coinages on a Latin model. Coined in 1933 by U.S. chemist Harold C. Urey (1893-1981).

The number of → deuterium (D) atoms with respect to → hydrogen (H) in an astrophysical object. Deuterium is a primordial product of → Big Bang nucleosynthesis. According to theoretical models, the primordial D/H ratio is estimated to be (2.61 ± 0.15) x 10^-5 (Steigman et al. 2007, MNRAS 378, 576). Nuclear reactions in stars convert D into He tending to a lower D/H ratio in the → interstellar medium over time (→ deuterium burning). However, chemical and physical → fractionation processes can produce local → enhancements in the D/H ratio. For example, low-temperature ion-molecule reactions in → molecular cloud cores can enhance the D/H ratio in icy grains by as much as two orders of magnitude above that observed in the interstellar medium.

The D/H ratio in the → solar nebula, estimated from observations of CH₄ in → Jupiter and → Saturn, is 2.1 ± 0.4 x 10^-5,
assuming that these gaseous planets obtained most of their hydrogen directly from solar nebula gas. This estimate is consistent with → protosolar D/H value inferred from the → solar wind implanted into lunar soils. Moreover, the D/H ratio derived from the interstellar Dα line (which is displaced from the → Lyman alpha line of ¹H at 1216 Å by -0.33 Å) is 1.6 x 10^-5 (Linsky et al. 1995, ApJ 451, 335).

High D/H ratios (relative to Earth’s water) are measured spectroscopically from water in three comets (all from the → Oort cloud):
→ Halley (3.2 ± 0.1 x 10^-4),
→ Hyakutake (2.9 ± 1.0 x 10^-4), and → Hale-Bopp (3.3 ± 0.8 x 10^-4). These are all about twice the D/H ratio for terrestrial water (1.49 x 10^-4) and about 15 times the value for the above-mentioned solar nebula gas. Note that
→ carbonaceous chondrites have the highest water abundance of all → meteorites. Their D/H ratios range from 1.20 x 10^-4 to 3.2 x10^-4 with a case at (7.3 ± 1.2) x10^-4.

Different authors interpret the high comet ratios in very different ways. Some consider the high D/H ratio as evidence against a cometary origin of most of the terrestrial water. Others, on the contrary, argue that comets are the main reservoir of deuterium-rich water that raised the terrestrial D/H a factor of six above the protosolar value.

For more details see “Sources of Terrestrial and Martian Water” by Campins, H. and Drake, M. (2010) in “Water & life: the unique properties of H20” Eds. R. Lynden-Bell et al. CRC Press, pp. 221- 234.

See also: → deuterium; → abundance.

The fusion of a deuterium nucleus with a proton which produces the lightest isotope of helium: D + H → ³He + γ. Deuterium burning occurs in stellar cores at a temperature exceeding
10⁶ K. The onset of deuterium burning marks the end of the → protostellar collapse. It is the only → nuclear reaction that occurs in → brown dwarfs. In normal stars, it is the second step in the → proton-proton chain which leads to the formation of ⁴He, allowing stars to arrive on the → main sequence.

See also: → deuterium; → burning.

The → enrichment of deuterium (D) with respect to → hydrogen (H) in
→ Solar System molecules
when compared with the D/H ratio in the → interstellar → solar nebula. H-bearing molecules in → comets, → planets, and → chondrite 
→ meteorites show a systematic D enrichment relative to the → molecular hydrogen of the solar nebula. Because there is no nuclear source for D in the Universe,
the observed → isotopic enrichment must have its origin in chemical reactions having faster reaction rates for D than for H.

In the Solar nebula the → isotopic fractionation of D between → water and H followed the reversible reaction:

H₂O + HD ⇔ HDO + H₂.

At low temperatures, this reaction favors the concentration of D in HDO. In the → interstellar medium grain chemistry plays a crucial role in D enrichment. See also → enrichment factor.

Apart from → deuterium fractionation, D could be enriched through another mechanism. Since molecular hydrogen (H₂) is more → volatile than molecular deuterium (D₂), D/H ratio could increase in certain planets that orbit near their star.

See also: → deuterium; → enrichment.

The ratio between the D/H value in → water and in → molecular hydrogen, as expressed by:
f = [(1/2)HDO/H₂O]/[(1/2)HD/H₂] = (D/H)_H2O/(D/H)_H2.

When f> 1, there is → deuterium enrichment.

See also: → deuterium; → enrichment; → factor.

The difference between the deuterium (D)/hydrogen (H)
→ abundance → ratio in an object with respect to that representing a standard or mean value for that type of objects. Same as → isotope fractionation of deuterium. In the gas phase chemistry many of the D fractionation reactions produce an excess of D atoms relative to → hydrogen atoms. Deuterium fractionation in → interstellar cloud cores, → protostars, and → Solar System bodies is frequently used to infer important aspects of their physical and chemical histories. For example, the → deuterium enhancement in the Earth’s sea water, with respect to the cosmic abundance, has been interpreted as being due to → enrichment by → comet-like → planetesimals colliding with the young Earth.

See also: → deuterium; → fractionation.

A nucleus of a deuterium atom (a combination of a proton and a neutron).

Etymology (EN): From Gk. deutero-, combining form of deuterios “second” + -ion a suffix used in the names of subatomic particles.

1. To elaborate or expand in detail, for example a theory.
   

2. To bring to a more advanced or effective state.
   

3. Math.: To express in an extended form, as in a → series. → developed turbulence, → development.

Etymology (EN): From M.Fr. développer, O.Fr. desveloper, from des- “undo” prefix + veloper “wrap up,” of unknown origin.

Etymology (PE): Govâlidan, from Proto-Iranian *vi-uar, cf. Skt. vi-vardh- “to grow up; to blossom,” Av. *vi-varəd-, from Skt./Av. prefix vi- “out, apart” + varəd- “to increase, augment, strengthen, cause to prosper,” Mid.Pers. vâlitan, Mod.Pers. bâlidan “to grow, to wax great.”