An Etymological Dictionary of Astronomy and Astrophysics

One of the pair of yellow lines in emission spectra of neutral sodium (Na I). D1 has a wavelength of 5895.94 Å and D2 is 5889.97 Å. This sodium doublet is one of the strongest absorption features in the spectra of late-type stars.

See also: Labelled D in a sequence of alphabetical letters first used by Joseph von Fraunhofer to designate spectral features in the solar spectrum, → Fraunhofer line.

The inner → Saturn’s rings, with a width of 7,500 km, lying before the → C ring, at 66,900 km from the center of Saturn.

See also: → ring.

An → ionization front of → H II regions
whose expansion speed is comparable to the → sound speed in the gas (~ 10 km/sec for hydrogen at 10⁴ K). A D-type ionization front results from → R-type ionization front when its propagation speed decreases as the volume of gas ahead of the ionization front grows. If front velocity is equal to a lower limit (C₁² / 2C₂, where C₁ and C₂ are the sound speed ahead and behind the front respectively), the front is called D critical.

See also: D referring to a dense gas; → type; → ionization; → front.

→ d’Alembert’s principle.

Etymology (EN): → d’Alembert’s principle; → Lagrangian.

A hydrodynamical paradox arising from the neglect of → viscosity in the → steady flow of a fluid around a submerged solid body. According to this paradox, the submerged body would offer no resistance to the flow of an → inviscid fluid and the pressure on the surface of the body would be symmetrically distributed about the body. This paradox may be traced to the neglect of the viscous forces, which are indirectly responsible for fluid resistance by modifying the velocity field close to a solid body (Meteorology Glossary, American Meteorological Society).

See also: → d’Alembert’s principle; → paradox.

The statement that a moving body can be brought to a → static equilibrium by applying an imaginary inertia force of the same magnitude as that of the accelerating force but in the opposite direction. More specifically, when a body of mass m is moving with a uniform acceleration a under the action of an external force F, we can write: F = m . a, according to Newton’s second law. This equation can also be written as: F - ma = 0. Therefore, by applying the force -ma, the body will be considered in equilibrium as the sum of all forces acting on it is zero. Such equilibrium is called → dynamic equilibrium. Owing to this principle, dynamical problems can be treated as if they were statical.

See also: Named after the French mathematician and philosopher Jean le Rond d’Alembert (1717-1783), who introduced the principle in his Traité de dynamique (1743).

A second order, → partial differential operator in space-time, defined as: &#9643² = ∂²/∂x² + ∂²/∂y² + ∂²/∂z² - (1/c²)∂²/∂t², or &#9643² = ∇² - (1/c²)(∂²/∂t²), where ∇² is the → Laplacian and c is the → speed of light. This operator is the square of the → four-dimensional operator 
&#9643, which is Lorentz invariant.

See also: → d’Alembert’s principle; → operator.

The → neutral → helium  → spectral line at 5876 Å.

See also: D3, because of confusion with the sodium → D lines.
When Joseph N. Lockyer first observed this line in the solar spectrum at the eclipse of 1868, helium was not yet isolated on Earth. Initially,
this line was thought to be the third member of the D1 and D2 line family
of sodium which lie in the same yellow part of the spectrum; → line.

A → white dwarf whose spectrum shows the → Balmer lines of hydrogen only, with no helium or metals.

See also: D short for → dwarf; A a convention; → white.

A 40-year period of unusually low → solar activity, from about 1790 to 1830. See also the → Maunder minimum.

See also: Named after John Dalton (1766-1844), British meteorologist; → minimum.

Of an oscillating system, the progressive decrease with time in the amplitude of the oscillation, due to friction (internal or external) or other resistance.

Etymology (EN): M.E. damp “black damp, a poison gas occurring in a mine,” from Mid.Du. or Mid. Low German; akin to O.H.G. damph “vapor.”

Etymology (PE): Mirâyi, noun from mirâ “getting extinguished, going out, expiring, dying,” from mordan, present tense 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,” Skt. mar- “to die,” mrti- “death;” 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 “man.”

1a) A successive group of rhythmical steps or bodily motions, or both, usually executed to music.

1b) An act or round of dancing.

2a) To move one’s feet or body, or both, rhythmically in a pattern of steps, especially to the accompaniment of music.

2b) To leap, skip, etc., as from excitement or emotion; move nimbly or quickly (Dictionary.com).

Etymology (EN): M.E. da(u)ncen “to dance,” from O.Fr. dancier of unknown origin, perhaps related to O.H.G. *dansjan “to lead (someone) to a dance.”

Etymology (PE): Vašt, variant of gašt, gardidan, gel, gelidan “to turn,” → revolve, cf.
Eastern Gilâni gilâr “dance.”

1. A person who dances.
   

   2. A person who dances professionally, as on the stage (Dictionary.com).

Etymology (EN): → dance; → -er.

Etymology (PE): Vaštâr, from vašt “dnace,” + agent noun suffix -âr, as in parastâr, padidâr; vaštande, vaštgar with agent noun suffixes, -ande and -gar, → -or.

1. Liability or exposure to harm or injury; risk; peril.
   

2. An instance or cause of peril; menace (Dictionary.com).

Etymology (EN): M.E. daunger, from O.Fr. dangier “power, power to harm, authority, control,” alteration of dongier, from V.L. *dominarium “power of a lord,” from L. dominus “lord, master,” → domain.

Etymology (PE): Xatar “danger,” loan from Ar.

A modern unportable astrolabe which is used for high precision measuring of stellar and geographical coordinates. The instrument
uses the simultaneous observations of two images of the same star, one of the images
formed directly by the lower face of a prism and the other by the light rays reflected first from a mercury bath and then by the upper face of the prism. The images coincide when the zenithal distance of the star attains a prefixed value (Gauss method of equal altitudes, → almucantar). Apart from astrometry, the Danjon astrolabe was used for studying the Earth’s rotation and is currently used for solar radius measurements.

See also: After André Danjon (1890-1967), French astronomer, who developed the instrument at the Strasbourg Observatory before the Second World War and at the Paris Observatory in 1948. The concept of prism astrolabe was initially invented by the French Auguste Claude (1858-1938) around 1900 and was later modified in collaboration with Ludovic Driencourt (1861-1940); → astrolabe.

A scale to evaluate as exactly as possible the darkening degree of a total → lunar eclipse. The five steps of the scale run from 0 (extremely dark, invisible Moon) to 4 (extremely bright, the eclipse having a very weak effect on the Moon’s visibility). The darkening at a lunar eclipse is determined to a great extent by the transparency of the terrestrial atmosphere, which is affected by clouds and the dust from the volcanic eruptions (M.S.: SDE).

See also: Named after André Danjon, who set up the scale, → Danjon astrolabe; → scale.

1. To be courageous enough to try to do something.
   

   2. A challenge, especially to prove courage.

Etymology (EN): M.E. durren, from O.E. durran “be bold enough, have courage” (to do something); cf. O.Norse dearr, O.H.G. giturran, Gothic gadaursan, from PIE root *dhers- “bold” source also of O.Pers. darš-, as below.

Etymology (PE): Daršidan, from O.Pers. darš- “to dare,” Av. darš- (prefixed *upa- in upadarəš- “to dare”); cf. Khotanese darv- “to dare;” Baluci durrit/durr- “to take courage;” Skt. dhars “to venture;” Gk. thrasus “bold;” Goth. ga-daursan “to venture;” E. “to dare;” PIE *d^hers- “to attack, venture, dare” (Cheung 2007).

Taking or willing to take risks; audaciousness.

See also: → dare; → -ing.

Having very little or no light.

Etymology (EN): M.E. derk, O.E. deorc, from P.Gmc. *derkaz.

Etymology (PE): Târik, Mid.Pers. târig “dark,” târ “darkness,” Av. taθra- “darkness,” taθrya- “dark,” cf. Skt. támisrâ- “darkness, dark night,” L. tenebrae “darkness,” Hittite taš(u)uant- “blind,” O.H.G. demar “twilight.”

The automatic adjustment of the iris and retina of the eye to allow maximum vision in the dark, following exposure of the eye to a relatively brighter illumination.

See also: → dark; → adaptation.

A relatively dense cloud of → interstellar gas, mainly molecular, whose dust particles obscure the light of stars behind it. A famous example is the → Horsehead Nebula silhouetted against the reddish glow of the → H II region IC 434. Individual dark clouds come in a range of sizes from tens of → light-years to tiny → Bok globules of only a few thousands → astronomical units.

See also: → dark; → cloud.

Current generated in an electronic detector by thermal effects, even in the absence of input signal. In a → CCD detector, the current rises from thermal energy within the silicon lattice comprising the CCD. These electrons are captured by the CCD’s potential wells and counted as signal. → dark current noise.

See also: → dark; → current.

In a → CCD detector, statistical fluctuation of the → dark current, equal to the square root of the dark current. CCDs can be cooled either with thermoelectric coolers or liquid nitrogen to reduce this effect. Ideally, the dark current noise should be reduced to a point where its contribution is negligible over a typical exposure time.

See also: → dark; → current; → noise.

A hypothetical form of energy that fills all the space and tends to increase the rate of expansion of the Universe. Assuming the existence of dark energy is a way to explain recent observations that the Universe appears to be expanding at an increasing rate (→ accelerating Universe). Dark energy seems to be a kind of anti-gravity force and is supposed to be related to → vacuum energy. Where gravity pulls things together at the more local level, dark energy tears them apart on the grander scale. The acceleration equation, one of Einstein’s equations for the homogeneous Universe, indicates that if the Universe is accelerating, the pressure of the driving component should be strongly negative.
The dark energy density relates to the → cosmological constant via: ρ_&Lambda = Λc²/(8πG), where G is the → gravitational constant and c the → speed of light. The first indication of dark energy was provided by the observation of → Type Ia supernovae. Other probes of dark energy are: → baryon acoustic oscillations, → weak gravitational lensing, and clusters of galaxies. In the standard model of cosmology, dark energy currently accounts for almost 74% of the total mass-energy of the Universe.
Two proposed forms for dark energy are the cosmological constant and exotic component such as → quintessence.

See also: → dark; → energy.

A → CCD frame obtained with closed → shutters in the absence of any light source, in order to estimate the → dark current of the → detector.

See also: → dark; → exposure.

One of the successive dark and light bands produced by diffraction or interference of light.

Etymology (EN): From O.Fr. frange, from V.L. *frimbia alteration by metathesis of L. fimbria “fringe, edge of garment.”

Etymology (PE): Fariz, variants farviz, farâviz “fringe, edge of garment.”

Matter that has no radiation and therefore cannot be detected directly,
but whose presence can be inferred from dynamical phenomena produced by its gravitational influence. The existence of dark matter is deduced mainly from the rotational speeds of galaxies, velocities of galaxies in clusters, gravitational lensing by galaxy clusters, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter plays also a central role in cosmic structure formation. There exists a large number of → non-baryonic dark matter candidates. They include, the hypothetical stable particles → WIMPs, → neutralinos, → axions,
→ gravitinos, etc. Among unstable candidates are gravitinos with mild R-parity violation and sterile neutrinos. See also → baryonic dark matter, → dark matter candidate.

See also: The concept of dark matter was first introduced by J.H. Oort (1932, Bull. Astron. Inst. Netherlands, 6, 249), who studied the vertical motions of the stars in the solar neighborhood and found that the visible matter could account for at most 50% of the derived surface density. → dark; → matter.

A hypothetical process whereby hypothetical → non-baryonic dark matter particles undergo → annihilation interactions with themselves. The process results in observable by-products such as high-energy photons, neutrinos, and other detectable particles. See also
→ dark matter decay.

See also: → dark; → matter; → annihilation.

A hypothetical physical entity capable of accounting for the corresponding observed phenomena involving → dark matter. → Non-baryonic dark matter candidates include → WIMPs, → neutralinos, → axions, → gravitinos. Among → baryonic dark matter
candidates can be noted ordinary and heavy → neutrinos, clouds of → neutral hydrogen gas, and compact objects.

See also: → dark; → matter; → candidate.

In theoretical models, the hypothetical transformation of a → non-baryonic dark matter particle when symmetry is violated at special physical conditions. Dark matter decay and → dark matter annihilation are expected to produce enormous amounts of energy in the form of gamma-rays, cosmic rays, etc.

See also: → dark; → matter; → decay.

A vast region surrounding a galaxy where dynamical tracers reveal a large
amount of → hidden mass. The halo has considerable mass but relatively low luminosity, suggesting the presence of a lot of → dark matter.

See also: → dark; → matter; → halo.

An interstellar cloud of absorbing matter whose dust particles obscure the light from stars beyond it and give the cloud the appearance of a dark, starless region.

See also: → dark; → nebula.

A hypothetical stellar object whose structure or evolution has been affected by → dark matter. Dark matter models predict that in the → early Universe dark matter (in the form of → WIMPs) should congregate and annihilate in the cores of → Population III stars. The result would be dark stars with properties very different from ordinary stars. The reason is that the presence of large amounts of dark matter during the formation of a star inhibits the collapse and can partially prevent further cooling of the gas beyond a certain critical point, well before a → main sequence object has formed. As a consequence, dark matter stars should be more massive, more luminous, and live longer than Pop. III stars, but would be cooler. The formation of dark stars could have had an
impact on the → reionization history of the Universe (see, e.g., P. Scott, 2011, astro-ph/1101.1029, and references therein).

See also: → dark; → star.

To become dark or darker. → darkening.

See also: Verb of → dark.

The act or process of becoming dark or darker. → limb darkening.

See also: Verbal noun of of → darken.

Items of information.

Etymology (EN): L. data, plural of → datum.

Etymology (PE): Dâdehâ, plural of dâdé, → datum.

In the → software architecture, the code that deals with reading from or writing to the data store, hiding its nature and complexity.

See also: → data; → access; → layer.

The process of controlling telescope operations during observation and obtaining data.

Etymology (EN): → data; acquisition, from L. acquisitionem, from acquirere “get in addition,” from → ad- “extra” + quærere “to search for, obtain.”

Etymology (PE): Alfanješ, verbal noun of alfanjidan (variant alfaqdan) “to acquire, get,” Bactrian αλφανζ “to acquire,” Sogdian δβ’yz “to acquire, gain, get” (Cheung 2007); → data.

The evaluation of reduced data; → data reduction.

See also: → data; → analysis.

Any extensive record or collection of data, observational (usually obtained with a particular instrument) or theoretical (grid of models usually regarding a particular branch of astrophysics).

See also: → data; → archive.

A set of data related to a given subject and organized in such a way that it can be consulted by users.

Etymology (EN): → data; bank, from M.Fr. banque, from O.It. banca “table,” because of “the moneylender’s exchange table,” from P.Gmc. *bankiz (O.H.G. bank “bench”).

Etymology (PE): Bânk, from Fr. banque, see above; dâdehâ, plural of dâdé→ datum.

The process of digging through large quantities of raw data stored in → databases to uncover new characteristics, patterns, and correlations using various automatic or semi-automatic statistical/mathematical means.

Etymology (EN): → data; mining, from mine, M.E.,
from O.Fr. mine, probably from a Celtic source.

Etymology (PE): Kânkâvi, literally “digging mine,” from kân “mine,” related to kandan “to dig” (Mid.Pers. kan, qn; O.Pers. ka(n) “to dig;” Av. kan- “to dig;” cf. Skt. khan- “to dig”) + kâvi from kâvidan/kâftan “to split; to dig” (Mid.Pers. kandan “to dig;” cf. Gk. skaptein “to dig;” L. scabere “to scratch, rub”).

An abstract entity that describes the structure of → database by including the formal description of the information system used in the database.

See also: → data; → model.

Unit of the analog-to-digital conversion system of a CCD apparatus. For example a 16 bit system may use a maximum of 65536 DN. The acronym ADU, for analog-to-digital unit, is also used.

See also: → data; → number.

Systematic operations on data, such as handling, merging, sorting, and computing.

See also: → data; → processing.

A machine for handling data in → data processing.

See also: → data; → processor.

The process of converting crude observational data into usable information for scientific interpretation, by correcting, rearranging, ordering, and simplifying.

See also: → data; → reduction.

A → method or → format for organizing and storing data. Any data structure is designed to organize data to suit a specific purpose so that it can be accessed and worked with in appropriate ways. In computer programming, a data structure may be selected or designed to store data for the purpose of working on it with various algorithms.

See also: → data; → structure.

A combined and coordinated set of data that supplies information for a specific purpose in a variety of forms.

See also: → data; → base.

A particular day, month, and year at which some event happened or will happen.

Etymology (EN): Date, from O.Fr. date, from M.L. data, from datus “given,” p.p. of dare “to give, grant, offer,” from PIE base *do- “to give” (cf. Pers. dâdan “to give,” as below).
The Roman convention of closing a document by writing “given” and the day and month (meaning “given to messenger”) led to data becoming a term for “the time stated.”

Etymology (PE): Gâhdâd, from gâh “time” (Mid.Pers. gâh, gâs “time;” O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come”) + dâd “given,” as in Latin;
p.p. of dâdan “to give” (Mid.Pers. dâdan “to give,” O.Pers./Av. dā- “to give, grant, yield,” Av. dadāiti “he gives,” Skt. dadáti “he gives,” Gk. didomi “I give;” akin to L. data, as above); cf. Mid.Pers., Mod.Pers. dâd “year, age, period of life,” Lori, Laki dâ(d) “age,” homdâ “of equal age.”
Note: In current Persian a single term, târix (تاریخ), is used for two different but related concepts: date and history. This may be confusing, → history (تاریخ).

Use of appropriate techniques to estimate the age of geological specimens or astronomical objects such as meteorites.

Etymology (EN): Verbal noun of date, from O.Fr. date, from M.L. → data.

Etymology (PE): Sen yâbi, from Ar. sen “age” + yâbi verbal noun of
yâftan, yâbidan “to find, discover; to obtain, acquire,” Mid.Pers. ayâftan, ayâpênitan “to reach, attain,” Manichean Mid.Pers. ‘y’b “to attain,” Parthian, Sogdian (+ *pati-) pty’b “to reach, obtain,” Av. ap- “to reach, overtake,” apayeiti “achieved, reached,” Skt. âp- “to reach, gain,”
âpnoti “reaches, gains,” Gk. hapto, haptomai “to touch, cling to, adhere to,” L. apiscor “touch, reach;” PIE base *ap- “to take, reach.”

The form of a word (in Latin and other inflected languages) indicating the noun or pronoun for which an action happens or a quality exists. In English loosely used for → indirect object (for example, him in Give him the book).

Etymology (EN): From L. casus dativus (“case for giving”), a translation of Gk. dotike ptosis (“inflection for giving”), from dativus “pertaining to giving,” from datus “given” (from PIE root *do- “to give”), → datum; → case.

Etymology (PE): Kâte, → case; barâyi, adj. of barâye, “→ because of,” “→ for.”

1. A single piece of information, as a fact, statistic, or code.
   

2. Any numerical or geometric quantity which serves as a reference or a base for measurement of other quantities.
   

3. Surveying: Any basis level (surface, line, or point) used as a reference in measuring elevations.

Etymology (EN): L. datum “given,” neuter p.p. of dare “to give, offer,” cf. Av. and O.Pers. dā- “to give, grant, yield,” Av. dadāiti “he gives,” Skt. dadáti “he gives,” Gk. didomi “I give,” PIE base *do- “to give.”

Etymology (PE): Dâdé “given,” p.p. of dâdan “to give,” Mid.Pers. dâdan “to give,” O.Pers./Av. dā- “to give, grant, yield,” akin to L. data, as above.
Dâdebon, from dâdé “datum,” + bon “basis, base,” → element.
Farâzbon, from farâz “above, up, upon,” → height,

• bon “basis, base,” as above.

A term used in → nuclear physics to denote a → nuclide produced by the → radioactive decay of a → parent nuclide.

Etymology (EN): From M.E. doughter, O.E. dohtor, from P.Gmc. *dochter
(Ger. Tochter), from PIE *dhug(h)əter-; cf. Av. dugədar-, duγdar-, Mod.Pers. doxtar,
Skt. duhitár-, Gk. thygater, Arm. dowstr, Lith. dukte.

Etymology (PE): Doxtar, from Mid.Pers. duxtar, duxt, O.Pers. *duxçi-, Av. dugədar-, duγdar-, cognate with daughter, as above.

In → nuclear physics, same as → daughter.

See also: → daughter; → product.

The experiment carried out in 1927 that confirmed the
→ de Broglie hypothesis as to the → wave nature of the → electron. It showed that electrons scattering off crystals form a → diffraction pattern. The experimental setup consisted of a → nickle chloride → crystal as → target, an electron gun, and a → detector placed on a graduated circular scale. The intensity of the reflected electrons was measured as a function of angle and electron energy. The observations showed a strong intensity peak at a certain angle. The nickel crystal acted as a → diffraction grating. → Constructive interference occurred
at a particular angle, where the peak intensity was observed in accord with → Bragg’s law. Interestingly, the intent of the initial experiment was was not to confirm the de Broglie hypothesis. In fact, the discovery was made by accident.

See also: Carried out by American physicists Clinton Davisson (1881-1958) and Lester Germer (1896-1971); → experiment.

The first daylight in the sky before sunrise, equivalent to morning astronomical → twilight; opposed to → dusk.

Etymology (EN): M.E. dawen (v.), from O.E. dagung, from dagian “to become day,” from root of dæg→ day.

Etymology (PE): Pegâh, from Mid.Pers. pa gâh, literally “soon; near the time (of sunrise),” from pa “to; for; in; on; with; by; according to,” O.Pers. upā, Av. upa “toward, with, on, in, in the time of” (cf. Skt. úpa “toward, together with, under, near to, on,” Gk. hypo “under, below,” L. sub “under,” Ger. auf, E. up; PIE *upo “under, up from under, over”) + gâh “time,” O.Pers. gāθu-, Av. gātav-, gātu- “place, throne, spot” (Skt. gátu- “going, motion; free space for moving; place of abode,” PIE *gwem- “to go, come”).
Sepidedam, from sepidé “whitish; dawn,” from sepid “white” (Mid.Pers. spêt, Av. spita- “white,” Skt. śveta- “white,” Lith. sviesti “to shine,” svaityti “to brighten,” P.Gmc. *khwitaz, O.E. hwit, E. white, Ger. weiss, PIE *kwintos/*kwindos “bright”) + dam “moment, time; to blow, breathe; smoke” (Mod./Mid.Pers. damidan “to blow, breathe,” Av. dāδmainya- “blowing up,”
Skt. dahm- “to blow,” dhámati “blows,” Gk. themeros “austere, dark-looking,” Lith. dumti “to blow,” PIE dhem-/dhemə- “to smoke, to blow”).
Bâmdâd, from Mid.Pers. bâmdât, Av. bāmya- “the dawn,” from bā- “to shine” (cf. Skt. bhā- “light, brightness,” Gk. phainomai “to appear, shine, gleam,” Arm. banam “to open, uncover,” PIE *bhā- “to shine”).

NASA’s mission to explore the two largest objects in the → asteroid belt, the asteroid Vesta and the → dawarf planet Ceres, gathering data relating to their composition, internal structure, density and shape. Launched in September 2007, Dawn entered the orbit of → Vesta in July 2011 and spent 16 months there before leaving for → Ceres. It entered Ceres orbit on March 6, 2015. The Dawn spacecraft is made of aluminium and graphite composite, it has a dry mass of 747.1 kg and a mass of 1217.7 kg when fully fuelled prior to launch. The spacecraft is a box-shaped design measuring 1.64m × 1.27m × 1.77m. Dawn ran out of fuel on Nov. 1, 2018, but the spacecraft will continue to orbit Ceres for several decades.

See also: → dawn; → spacecraft.

The length of time it takes Earth (or a planet) to rotate once on its axis relative to some external reference. The day is measured in several ways, depending on this reference, → sidereal day; → solar day; → mean solar day.

Etymology (EN): M.E., from O.E. dæg (cf. Ger. Tag, Swedish and Danish dag “day”), from PIE base *dhegh- “to burn.” Not related to L. dies “day” (from *dyeu- “to shine,” → diurnal), but rather to Av. dag- “to burn,” dažaiti “burns,” Mod.Pers. dâq “hot,” Skt. dah- “to burn,” dáhati, Gk. tephra “ash,” L. fovere “to boil,” Albanian djeg “to burn,” Russ. žeč’ “to burn,” Lith. dagas “hot season,” degti “to burn,” O.Prus. dagis “summer.”

Etymology (PE): Ruz “day,” from Mid.Pers. rôc, O.Pers. raucah-, Av. raocah- “light, luminous; daylight,” Skt. roka- “brightness, light,” cognate with Gk. leukos “white, clear,” L. lux “light” (also lumen, luna), PIE base *leuk- “light, brightness”. The Persian words rowšan “bright, clear,” foruq “light,” and afruxtan “to light, kindle” also belong to this family, as well as the E. light, Ger. Licht, and Fr. lumière.

A → meteor detected using → radar techniques during daylight or when skies are cloudy.

See also: → day; → light; → meteor.

A system of adjusting the official local time in some countries in order to provide a better match between the hours of daylight and the active hours of work and school. The “saved” daylight is spent on evening activities which get more daylight, rather than being “wasted” while people sleep past dawn. Although known also as summer time, it includes the spring season and nearly half of autumn.

Etymology (EN): → day; → light; saving, from save, from O.Fr. sauver, from L.L. salvare “to secure,” from L. salvus “safe,” PIE *solwos, from base *sol- “whole” (cf. O.Pers. haruva-, Av. haurva- “whole, intact,” Mod.Pers. har “every, all; any,” Skt. sarva- “whole, entire,” Gk. holos “whole”); → time.

Etymology (PE): Vaxt, written vaqtوقت but pronounced vaxtوخت, is a Pers. word meaning “portion (of time)”. Its variants and related words in Mod./Mid.Pers. are: baxt “what is alloted, fate, fortune,” baxš “portion, part, division,” baxšidan, baxtan “to divide, distribute, grant,” Av. base bag- “to attribute, allot, distribute,” baxš- “to apportion, divide, give to,”
baxta- “what is alloted (luck, fortune),” baxədra- “part, portion,” baγa- “master, god,” O.Pers. bāji- “tribute, tax,” cf. Skt. bhaj- “to share, divide, distribute, apportion,” bhájati “divides,” bhakta- “alloted; occupied with; a share; food or a meal, time of eating?,” Gk. phagein “to eat (to have a share of food)”; PIE base *bhag- “to share out, apportion.”
nur→ light.
anduzi, verbal noun of anduxtan “to save; acquire, gather,” from Mid.Pers. handôxtan, handôz- “to gain, acquire, amass,” from *ham-tuj-, from ham- “together,” → com- + *tuj- “to save, gather, (re)pay,” cf. Skt. tuj- “to promote, be strong, move quickly.”

The time interval when the Sun is above the horizon for a given position.

Etymology (EN): → day; → time.

Etymology (PE): Ruzhangâm, from ruz→ day + hangâm “time, hour, season,” Mid.Pers. hangâm “time, epoch, season,” Av. ham-gam- “to meet together,” from ham- “together,” → com- + gam- “to come; to go,” jamaiti “goes,” O.Pers. gam- “to come; to go,” Mod.Pers. âmadan “to come,” Skt. gamati “goes,” Gk. bainein “to go, walk, step,” L. venire “to come,” Tocharian A käm- “to come,” O.H.G. queman “to come,” E. come; PIE root *gwem- “to go, come.”

The observed absence of helium white dwarfs with temperatures between 45,000 and 27,000 K. A possible explanation lies in the chemical evolution of white dwarfs. Four processes can change the structure of a white dwarf: gravitational settling, interstellar medium accretion, mass loss, and subsurface convective mixing.

See also: → DB white dwarf; → gap.

A → white dwarf whose spectrum shows strong He I in the absence of hydrogen or metal lines.

See also: D short for → dwarf; B a convention; → white.

A → white dwarf showing a continuous spectrum with no readily apparent lines.

See also: D short for → dwarf; C a convention; → white.

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.