An Etymological Dictionary of Astronomy and Astrophysics

A prefix meaning “two; twice; double.” → dimer, → diode, → dipole, → diproton.

See also: L. di, from Gk., → two.

To identify the nature of (an illness or other problem) by examination of the symptoms (OxfordDictionaries.com).

See also: → diagnosis.

1. The process of determining by examination the nature and circumstances of a diseased condition.
   

   2. The decision reached from such an examination.
      

   3. Scientific determination; a description that classifies a group or taxon precisely; a determining or analysis of the cause or nature of a problem or situation (Dictionary.com).

Etymology (EN): L., from Gk. diagnosis “a discerning, distinguishing,” from stem of diagignoskein “discern, distinguish,” literally “to know thoroughly,” from dia- “through” + gignoskein “to learn,” cognate with Pers. šenâs-, šenâxtan “to know, to be acquinted” and dânestan “to know,” as below, ultimately from PIE root *gno- “to know.”

Etymology (PE): Parnâs, from par- “through,” + nâs “to know,” as in Zazaki nâs- “to know,” Kurd. nâs-, nâsîn “to know,” related to Pers. šenâs- “to know,” related to šenâxtan, → knowledge and
dânestan, → science.

1a) Of, relating to, or used in → diagnosis.

1b) Serving to identify or characterize; being a precise indication.

2a) Diagnosis.

2b) A symptom or characteristic of value in diagnosis.

2c) A message output by a computer diagnosing an error in a computer program, computer system, or component device; a program or subroutine that produces such messages (Dictionary.com).

Etymology (EN): From Gk. diagnostikos “able to distinguish,” → diagnosis.

Etymology (PE): → diagnosis.

In a → polygon, a line segment joining any two non-adjacent vertices (→ vertex).

Etymology (EN): From M.Fr. diagonal, from L. diagonalis, from diagonus “slanting line,” from Gk. diagonios “from angle to angle,” from dia- “across, dividing two parts” + gonia “angle,” related to gony “knee,” L. genu “knee,” Mod.Pers. zânu “knee,” Av. žnav-, žnu- “knee,” Skt. janu-; PIE base *g(e)neu-, see below.

Etymology (PE): Tarâkonj, from tarâ- “across, through,” → trans-,

• konj “angle, corner, confined place” (variants xong “corner, angle,” Tabari kânj, Kurd. kunj, Hamadâni kom), maybe from the PIE base *g(e)neu-, as above, and related to Mod.Pers. zânu “knee” (Av. žnu-), Skt. kona- “angle, corner,” Gk. gony, gonia, L. cuneus “a wedge,” Albanian (Gheg dialect) kân “angle, corner,” Albanian (Toks) kënd “angle, corner.”

A graphic representation of the behavior of one or several variables.

Etymology (EN): From Fr. diagramme, from L. diagramma, from Gk. diagramma “that which is marked out by lines,” from diagraphein “to mark out by lines,” from dia- “across, out” + graphein “to write, draw,” → -graphy.

Etymology (PE): Nemudâr agent noun of nemudan “to show,” → display, from the past stem nemud + -âr, such as xâstâr, foruxtâr, padidâr, parastâr (contraction of *parastidâr).

Relative to or characterized by → diamagnetism.

See also: → diamagnetism.

The property of a substance, like bismuth, that creates a weak magnetic field in opposition of an externally applied magnetic field, thus causing a repulsive effect.
In diamagnetic materials the → magnetic moments of individual atoms are not permanent. Within each atom the electron spins and orbital motions all exactly balance out, so any particular atom has no net magnetic moment. The external magnetic field generates little currents by induction. According to → Lenz’s law, the induced magnetic moments of the atoms are directed opposite to the magnetic field.

Etymology (EN): Diamagnetic, from Gk. dia- a prefix used with several meanings “passing through; thoroughly; completely; going apart,” and in the present case “opposed;” → magnetic. magnetic.

Etymology (PE): Pâdmeqnât, from pâd- “against, contrary,” → anti-,

• megnât→ magnetism.

Any chord passing through the center of a figure. The length of this chord.

Etymology (EN): O.Fr. diamètre, from L. diametrus, from Gk. diametros “diagonal of a circle,” from dia- “across, through” + metron “a measure” → meter.

Etymology (PE): Tarâmun, from tarâ- “across, through,” → trans-

• mun/mân “measure,” as in Pers. terms pirâmun “perimeter,” âzmun “test, trial,”
  peymân “measuring, agreement,” peymâné “a measure; a cup, bowl,”
  from O.Pers./Av. mā(y)- “to measure,” cf. Skt. mati “measures,” matra- “measure,” Gk. metron “measure,” L. metrum; PIE base *me- “to measure.”

A crystalline form of → carbon, which is the hardest substance known. Each carbon in a diamond crystal is bonded to four other carbon atoms forming a tetrahedral unit. This tetrahedral bonding of five carbon atoms forms an incredibly strong molecule. Diamond has a very high → refractive index and → dispersive power. It is colorless when pure, and sometimes colored by traces of impurities. Natural diamond was formed billions of years ago within the Earth → mantle at depths greater than 150 km, where pressure is roughly 5 giga→ pascals and the temperature is around 1200 °C. Diamonds reach the surface of the Earth via volcanic eruptions. Similarly very small diamonds (micrometer and nanometer sizes) are usually found in impact sites of → meteorites.
Such impact events create shock zones of high pressure and temperature suitable for diamond formation. When diamond is exposed to high temperatures or ion bombardment, it will be transformed into → graphite.

Etymology (EN): Diamond, from O.Fr. diamant, from M.L. diamant-, diamas-, from L. adamant-, adamas “hardest metal,” from Gk. adamas “unbreakable,” from → a- “not” + daman “to subdue, to tame;” cognate with Pers. dâm “a tame animal.”

Etymology (PE): Almâs, loanword from Gk., as above.

An intense flash of light that happens a few seconds before and after totality during a solar eclipse. The effect is caused by the last rays of sunlight before totality (or the first rays of sunlight after totality) shining through valleys on the edge of the Moon.

Etymology (EN): → diamond; → ring; → effect.

Etymology (PE): Oskar, → effect; angoštar “a ring worn on the finger,” from angošt “finger,” Mid.Pers. angušt “finger, toe,” Av. angušta- “toe,” from ank- “curved, crooked,” cf. Skt. angustha- “thumb,” ankah “hook, bent,” Gk. angkon “elbow,” angkura “anchor,” L. angulum “corner” (E. angle),
Lith. anka “loop,” O.E. ancleo “ankle,” O.H.G. ango “hook,” PIE base *ang-/*ank- “to bend”; → diamond.

A device with a restricted aperture, located in an optical system at any of several points, that cuts off marginal light rays not essential to image formation.

Etymology (EN): From L.L. diaphragma, from Gk. diaphragma “partition, barrier,” from diaphrassein “to barricade,” from dia-
“across” + phrassein “to fence or hedge in.”

Etymology (PE): Miyânban, from miyân “middle, interior, between” (Mid.Pers. miyân “middle,” Av. maiδya-, maiδyāna- “medium, middle,” cf. Skt. mádhya- “middle, intemediate,” Gk. mesos “middle,” L. medius “middle,” Goth. midjis, O.E. midd “middle,” O.C.S. medzu “between,” Arm. mej “middle,” PIE *medhyo- “middle,” , from base *me- “between”) + band “barrier, shutter,” from
bastan “to bind, shut” (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”).

1. General: A division in two parts or kinds that differ widely from or contradict each other.
   

2. Astro.: The phase of the Moon or a planet when half of its surface appears illuminated by the Sun.

Etymology (EN): From Gk. dichotomia “cutting in two,” from dicho- “apart, in two,” combining form of dicha “in two, asunder,” akin to → di-, + temnein “to cut.”

Etymology (PE): Dopâregi, from do→ two + pâré “piece, part, portion, fragment” (Mid.Pers. pârag “piece, part, portion; gift, offering, bribe;” Av. pāra- “debt,” from par- “to remunerate, equalize; to condemn;” PIE *per- “to sell, hand over, distribute; to assign;” cf. L. pars “part, piece, side, share,” portio “share, portion;” Gk. peprotai “it has been granted;” Skt. purti- “reward;” Hitt. pars-, parsiya- “to break, crumble”) + -(g)i a noun/state suffix.

An → interference filter used to selectively pass light of a small range of colors while reflecting other colors.

See also: → dichroism, → filter.

Property of some crystals in which radiation polarized in one plane relative to the crystalline axes is freely transmitted, but radiation polarized perpendicular to this is absorbed. Tourmaline is a natural mineral with this property; Polaroid is a synthetic dichroic substance.

Etymology (EN): From Gk. dichro(os), from di- “two,” → di-, + chroma “color” + -ism.

Etymology (PE): Dofâmi, from do “two,” → bi- + fâm “color,”

• -i noun suffix.

A reference source in print or electronic form containing words alphabetically arranged along with information about their forms, meanings, pronunciations, etymologies, etc.

Etymology (EN): M.L. dictionarium “collection of words and phrases,” from L. dictionarius “of words,” from dictio “word” from dic-, p.p. stem of L. dicere “speak, tell, say,” from PIE root *deik- “to point out;” cf. Av. daēs- “to show,” daēsa- “sign, omen;” cf. Skt. deś- “to show, point out;” → form.

Etymology (PE): Farhang, → culture.

A substance in which an electric field gives rise to no net flow of electric charge but to a displacement of charge in opposite directions. The displacement is usually small compared to atomic dimensions. Dielectrics differ from conductors in that they have no free electrons to move through the material under the influence of an electric field. Most insulating materials, e.g. air, porcelain, mica, glass, are dielectrics. A perfect vacuum would constitute a perfect dielectric. → diamagnetic.

Etymology (EN): Dielectric, from Gk. dia- a prefix used with several meanings “passing through; thoroughly; completely; going apart,” and in the present case “not, opposed;” → electric.

Etymology (PE): Nâbarg, from nâ- “not,” → un-,

• barq→ electricity.

Any internal combustion → engine in which → adiabatic compression raises the temperature of air high enough so that fuel ignites spontaneously when injected.

See also: After Rudolf Diesel (1858-1913), German engineer; → engine.

An → equation of state for → real gases which leads to the → van der Waals equation as a → first approximation. It is of the form P(V - b) [exp (a/VRT)] = RT, where P is the pressure, V is the volume, T is the thermodynamic temperature, R is the → gas constant, and a and b are the constants characteristic of the gas.

See also: Named after Conrad Dieterici (1858-1929), a German physicist; → equation.

To be unlike, dissimilar, or distinct in nature or qualities.

Etymology (EN): From O.Fr. différer, from L. differre “to set apart, differ,” from → dis- “away from” + ferre “to carry, bear” (cf. Av. baraiti “carries,” O.Pers. barantiy “they carry,” Mod.Pers. barad “carries,” Skt. bharati “carries,” Arm. berem “I carry,” Gk. pherein “to carry,” O.E., O.H.G. beran, Rus. brat’ “to take,” bremya “a burden”).

Etymology (PE): Degarsân “different,” from degar “another, other” (Mid.Pers. dit, ditikar “the other, the second,” O.Pers. duvitiya- “second,” Av. daibitya-, bitya- “second,” Skt. dvitiya- “second,” PIE *duitiio- “second”) + sân “manner, semblance” (variant sun, Mid.Pers. sân “manner, kind,” Sogdian šôné “career”) + budan “to be” (Mid.Pers. budan, from O.Pers./Av. bav- “to be; become, take place,” Av. buta- perf. ptcpl. pass., bavaiti “becomes,”
Skt. bhavati “becomes, happens,” bhavah “becoming; condition, state,” PIE *bheu- “to be, come into being, become;” cf. Gk. phu- “become,” phuein “to bring forth, make grow,” L. fui “I was” (perf. tense of esse), futurus “that is to be, future,” Ger. present first and second person sing. bin, bist, E. to be, O.Ir. bi’u “I am,” Lith. bu’ti “to be,” Rus. byt’ “to be”).

The quality or condition of being different.
Math.: the amount by which one quantity is greater or less than another.
Of two sets A and B, the set consisting of all elements of A which do not belong to B.

See also: Noun form of → differ.

Not alike in character or quality; differing; dissimilar.

See also: Adj. of → difference.

Of a mathematical function, the quality of being → differentiable.

See also: Noun from → differentiable.

Capable of being → differentiated. → differentiable function.

See also: From → differentiate + → -able.

Property of a mathematical function if it has a → derivative at a given point.

See also: From → differentiable; → function.

1. Noun.
   1a) General: A difference or the amount of difference, as in rate, cost, quantity, degree, or quality, between things that are comparable.
   

1b) Math.: An infinitesimally small change in a variable; a function of two variables that is obtained from a given function, y = f(x), and that expresses the increment in the given function as the derivative of the function, f’(x), times the increment in the independent variable, written as dy = f’(x) dx.

1c) Electronics: Referring to a circuit, device, or machine whose principle of operation depends on the difference between two opposing effects.

2. Adjective.
   2a) General: Of, relating to, or constituting a difference.
   

2b) Physics: Pertaining to or involving the difference of two or more motions, forces, etc.

2c) Math.: Pertaining to or involving a → derivative or derivatives.

Etymology (EN): From M.L. differentialis, from differenti(a),
→ difference, + → -al.

Etymology (PE): Degarsâné, from degarsân, → different + noun suffix -é.

The two branches of mathematics that make up the → calculus. → differential calculus; → integral calculus.

See also: → differential; → integral; → calculus.

A branch of calculus which is concerned with the instantaneous rate of change of quantities with respect to other quantities, or more precisely, the local behavior of functions. → integral calculus.

See also: → differential; → calculus.

An equation expressing a relationship between an → independent variable, x, an unknown → function, y = f(x), and its → derivatives. The general form of a differential equation is:
F(x, y, y’, y’’, …, y⁽ⁿ⁾) = 0, or F(x,y, dy/dx, d²y/dx², …, dⁿy/dxⁿ) = 0. See also: → ordinary differential equation; → partial differential equation; → linear differential equation; → exact differential equation; → first-order differential equation; → homogeneous linear differential equation; → nonhomogeneous linear differential equation; → differential equation with separated variables; → differential equation with separable variables.

See also: → differential; → equation.

A → differential equation of the form: M₁(x) N₁(y) dx + M₂(x) N₂(y) dy = 0, which can be reduced to a → differential equation with separated variables.

See also: → differential; → equation; → separate; → variable.

A → differentail equation that can be transformed into the form: M(x)dx + N(x)dy = 0.

See also: → differential; → equation; → separate; → variable.

The study of curved spaces using differential calculus.

See also: → differential; → geometry.

A device that is commonly used to measure the → seeing at optical astronomical sites. The DIMM delivers an estimate of the → Fried parameter based on measuring the variance of the differential image motion in two small apertures, usually cut out in a single larger telescope pupil by a mask. The DIMM concept was introduced by Stock & Keller (1960, in Stars and Stellar Systems, Vol. 1, ed. G. P. Kuiper & B. M. Middlehurst, p. 138), whereas its modern implementation was first described by Sarazin & Roddier (1990, A&A 227, 294).

See also: → differential; → image; → motion; → monitor.

A problem encountered in astronomical spectroscopy, which consists of a loss of light from some wavelengths due to → atmospheric dispersion. In simple terms, differential refraction means that at nonzero → zenith distances an object cannot be simultaneously placed at the same position within a → slit at all wavelengths. This problem becomes more important for increasing → airmass, larger → spectral range, and smaller → slitwidths. To remedy this drawback, the slit should always be oriented along a direction perpendicular to the horizon, since differential refraction occurs in that direction.

See also: → differential; → refraction.

1. Of a single body (such as a star or a gaseous planet), the axial rotation of equatorial latitudes faster than polar latitudes.
   

2. Of a galaxy, the orbiting of stars nearer the center faster than those at the edge.

See also: → differential; → rotation.

A system characterized by → differential rotation. In such a system the → angular velocity decreases as the distance from the rotation center increases.

See also: → differential; → rotating; → system.

General: To perceive or show the difference in or between.
Math.: To obtain the differential or the derivative of.

Etymology (EN): M.L. differentiatus “distinguished,” p.p. of differentiare.

Etymology (PE): Degarsânidan, verbal form of → difference.

A description of a planet’s interior which is composed of a rocky, dense inner core and a less dense outer crust.

See also: Differentiated, p.p. of → differentiate; → interior.

A meteorite that has distinctly separated stone, metal, and glass. It is derived from a differentiated parent body and hence not primitive. The parent body accreted surrounding material until it was large enough to start melting in the middle. The denser metals sank to the center and the stones and glasses floated to the top. A differentiated meteorite made completely of metal comes from the center of a parent meteoroid which was broken apart. → undifferenciated meteorite.

See also: Differentiated, p.p. of → differentiate; → meteorite.

1. Math.: The operation of finding the → derivative of a function.
   

2. Astro.: Process by which an originally homogeneous planetary or asteroidal body is separated into regions of different composition, such as core, mantle, and crust.

See also: Verbal noun of → differentiate.

Not easy to do or to understand; hard to deal with.

Etymology (EN): Back-formation from difficulty; L. difficilis. “hard,” from → dis- “not, away from” + facilis “easy to do,” from facere “to do,” → fact.

Etymology (PE): Došvâr, from Mid.Pers. dušwâr “difficult, disagreeable,” variant dušxwâr antonym of xwâr “easy; light; low; mean, abject;” with prefix duš- “ill, evil, bad,” → dys-.

Verbal form of → diffraction.

See also: → diffraction.

A wave property of light which allows it to curl around obstacles whose size is about that of the wavelength of the light. As a → wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form a → diffraction pattern.

Related terms:
→ diffusion; → dispersion; → distribution; → scatter; → scattering.

Etymology (EN): From Fr. diffraction, from Mod.L. diffractionem, from L. diffrac-, stem of diffringere “break in pieces,” from → dis- “apart” + frangere “to break.”

Etymology (PE): Parâš “dispersion, scattering,” variant of pâš, pâšidan, → dispersion.

An optical device containing thousands of very fine parallel grooves which produce interference patterns in a way which separates out all the components of the light into a spectrum.

See also: → diffraction; → grating.

A series of concentric rings of dark and light color produced by interference.

Etymology (EN): → diffraction; → pattern.

Etymology (PE): Olgu, loanword from Turkish; parâš→ diffraction.

One of several light rays emanating from a bright light source in images taken with → reflecting telescopes. They are artifacts caused by light diffracting around the support or → spider vanes of the → secondary mirror.

See also: → diffraction; → spike.

The quality of an → optical system that is capable of producing images with angular resolution as small as the theoretical limit of the → Airy disk.

Etymology (EN): → diffraction; limited, adj. of
→ limit.

Etymology (PE): Karânmand “bounded, limited,” from karân→ boundary + -mand possession suffix; parâš→ diffraction.

1a) To pour out, to spread in all directions.

1b) To spread by → diffusion.

2. Spread out, diffused, scattered, as → diffuse reflection.
   

See also:

→ diffuse atomic cloud, → diffuse galactic light, → diffuse interstellar band, → diffuse interstellar band carrier, → diffuse interstellar cloud, → diffuse interstellar medium, → diffuse molecular cloud, → diffuse nebula, → diffuse reflection, → diffuse transmission, → diffusion.

Etymology (EN): L. diffusus “spread, poured forth,” from dif- “apart, in every direction,” variant of → dis- + fuse, from fusus “melted, poured, cast,” p.p. of fundere
“to melt, cast, pour out,” from PIE *gheud-, from root *gheu- “to pour.”

Etymology (PE): Paxšidan “to diffuse, scatter, disperse,” infinitive of paxš “scattered, dispersed; withered, trodden,” (Manichean) Mid.Pers. pxš “to wither, fade; to grow ripe,” Proto-Iranian *paxš- “to cook,” cf. Av. pac- “to cook,” pacika- “cooked,” Mod.Pers. paz-, poxtan “to cook, bake,” Skt. pac- “to cook,” pakva- “ripe,” Gk. peptein “to cook, ripen,” L. coquere “to cook,” from which V.L. cocus “cook,” from which O.E. coc “cook;”
PIE *pek^w- “to cook, ripen;” paxšidé, p.p. of paxšidan.

A type of cloud in the → interstellar medium with low molecular content that is fully exposed to the → interstellar radiation field, and therefore nearly all its → molecules are quickly destroyed by → photodissociation. Hydrogen is mainly in → neutral atomic form (→ neutral hydrogen), and atoms with → ionization potentials less than that of hydrogen (most notably → carbon) are almost fully → ionized, providing abundant electrons. The paucity of molecules implies that very little chemistry occurs in such clouds. Many → sightlines with low → extinction seem to pass exclusively through → diffuse atomic gas. Such sightlines typically have a → column density, N_H, less than about 5 × 10²⁰ cm^-2, and are sufficiently → optically thin to be observable by means of → visible and → ultraviolet → absorption line measurements. Diffuse atomic clouds typically have a fairly low → density (~ 10-100 cm^-3), and → temperatures of 30-100 K (Snow & McCall, 2006, ARA&A 44, 367).

See also: → diffuse; → atomic; → gas.

A minor component of galactic light resulting from the diffusion of starlight by → interstellar dust near the → galactic plane.

See also: → diffuse; → galactic; → light.

Absorption features in the spectrum of stars identified in the ultraviolet, visible, and infrared regions. They have an interstellar origin, but despite extensive efforts, their carrier(s) have not yet been clearly identified. See also → Aromatic Infrared Bands; → polycyclic aromatic hydrocarbons.

See also: → diffuse; → interstellar; → band.

The chemical element or composition that is supposedly at the origin of a → diffuse interstellar band (DIB).

See also: → diffuse; → interstellar; → band; → carrier.

An → interstellar cloud in which hydrogen is completely dissociated and which is less dense and dusty than → molecular clouds. In diffuse interstellar clouds photoabsorption of the background → ultraviolet (UV) radiation field is an important dissociating and ionizing process. Typical densities and temperatures of diffuse clouds are 10² to 10³ cm^-3 and 20 to 100 K respectively. Because of modest extinctions (≤ 1 mag), → photodissociation processes are important in diffuse clouds preventing the formation of larger molecules.

See also: → diffuse; → interstellar; → cloud.

The interstellar matter outside condensed molecular clouds.
Diffuse interstellar medium consists of a hot intercloud medium, a warm intercloud medium, and a cold neutral medium with hydrogen atom densities n_H ~ 0.003, ~ 0.25, and ~ 40 cm^-3, and mean gas → kinetic temperatures T_k ~ 5 x 10⁵, ~ 10⁴, and 80 K, respectively.

See also: → diffuse; → interstellar; → medium.

A type of → molecular cloud in which the → interstellar radiation field is sufficiently attenuated, so that the local fraction of → molecular hydrogen (H₂) becomes substantial (> 0.1). However, enough interstellar radiation is still present to → photoionize any atomic carbon, or to → photodissociate → carbon monoxide (CO) such that carbon is predominantly still in the form of C⁺ (> 0.5). In steady state, diffuse molecular clouds must necessarily be surrounded by diffuse atomic gas, in order to provide the → shielding of radiation. This means that most sightlines that cross a diffuse molecular cloud will also cross → diffuse atomic gas (Snow & McCall, 2006, ARA&A 44, 367).

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

An irregularly shaped and low density interstellar cloud visible in the optical wavelengths.

See also: → diffuse; → nebula.

Reflection of light from a rough or granular surface, which takes place in all directions due to the microscopic irregularities of the interface; opposed to → specular reflection.

See also: → diffuse; → reflection.

Transmission accompanied by diffusion or scatter to the extent that there is no regular or direct transmission.

See also: → diffuse; → transmission.

A device used to scatter or disperse light emitted from a source.

Etymology (EN): From → diffuse + -er.

Etymology (PE): From paxš, present stem of paxšidan, → diffuse, + -andé or -gar (→ detector).

1. Movement of a gas or liquid as a result of the random thermal motion of its atoms or molecules.
   

2. The random spreading out of a beam of radiation on reflection (→ diffuse reflection) or transmission through a translucent medium (such as amber, milk, or frosted glass).
   

3. In chemical thermodynamics, the process of attaining an equilibrium distribution of the → concentration within the phases. A result of diffusion at constant equilibrium is the equalization of the → chemical potentials.
   Related terms:
   → diffraction; → dispersion; → distribution; → scatter; → scattering.

Etymology (EN): L. diffusionem, from stem of diffundere “scatter, pour out,” from dif- “apart, in every direction,” → dis-,

• fundere “to melt, cast, pour out,” from PIE *gheud-, from root *gheu- “to pour.”

Etymology (PE): Paxš, verbal noun and stem of paxšidan→ diffuse.

A factor of proportionality involved in the → diffusion equation. It may be defined as the amount of the quantity diffusing across a unit area through a unit concentration gradient in unit time. → magnetic diffusivity.

See also: → diffusion; → coefficient.

An equation that expresses the time rate of change of a quantity in terms of the product of the diffusion coefficient and the → Laplacian operating on the quantity. For example the diffusion equation for temperature is:

∂T/∂t = D∇²T.

See also: → diffusion; → equation.

A narrow boundary layer above the solar
→ photosphere, between two magnetic field lines,
where the plasma becomes demagnetized or unfrozen. The presence of a localized magnetic region is necessary for → magnetic reconnection.

See also: → local; → -ize; → diffusion; → region.

Tending to diffuse; characterized by → diffusion.

See also: → diffuse + → -ive.

1. The ability to permit or undergo diffusion.
   

2. → magnetic diffusivity.

See also: → diffusive; → -ity.

A symbol, numeral, or graphic character that represents an integer.

Etymology (EN): From L. digitus “finger, toe.”

Etymology (PE): Raqam, from Ar.

1. Of or pertaining to a digit.
   

2. Representing data as a series of numerical values. Often opposed to
   → analogue.

See also: → digit + → -al.

A computer that accepts and operates with → discrete data in the form of combinations of digits, letters, or other characters. In modern terminology, generally called computer.

See also: → digital + → computer.

To convert an → analogue signal to a → digital signal that represents the same information as the analogue signal.

See also: → digit + → -ize.

1. Having formed by two half-planes which intersect.
   

2. Of or relating to a → dihedron.

See also: → dihedron; → -al.

An angle formed by two planes meeting along a common line. Compare with → face angle.

See also: → angle; → dihedral.

A figure formed by two intersecting planes.

See also: → di-; → -hedron.

1. (v.tr.) To make wider or larger; cause to expand.
   
2. (v.intr.) To spread out; expand.

Etymology (EN): M.E. dilaten, from O.Fr. dilater,
from L. dilatare “make wider, enlarge,” from → dis- “apart” + latus “wide,” → latitude.

Etymology (PE): Farâxândan, farâxidan, infinitive from farâx “broad, wide, spacious;” Mid.Pers. frâxv, fraxv “broad, wide,” frâxvitan, frâxvênitan “to spread.”

The act of dilating; state of being dilated. Also dilatation.
Physics: The increase in volume per unit volume of a homogeneous substance. → time dilation.

See also: Verbal noun of → dilate.

1. (adj.) Describing a solution that is reduced in concentration.
   
2. (v.tr.) To make a solution thinner by the addition of water or the like.

Etymology (EN): From L. dilutus, p.p. of diluere “dissolve, wash away,” from → dis- “apart” + -luere, combining form of lavere “to wash;” cf. Pers. lur “flood” [Mo’in, Dehxodâ]
(variants Lori, Kordi: laf, lafow, lafaw, Tabari: lé); Gk. louein “to wash;”
Bret. laouer “trough;” PIE base *lou- “to wash.”

Etymology (PE): Owtâl, from Tabari utâl, “having water, impregnated with water, waterlogged,” from ow “water,” → water + tâl variant of dâr “having, possessor,” from dâštan “to have, to possess” (Mid.Pers. dâštan;
O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind;” Skt. dhr-, dharma- “law;”
Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE base *dher- “to hold, support”).

The process of reducing the concentration of solute in a solution by increasing the proportion of solvent.

See also: Verbal noun of → dilute.

The energy density of a radiation field divided by the equilibrium value for the same color temperature.

See also: → dilution; → factor.

Not bright; obscure from lack of light.

Etymology (EN): O.E. dimm “dark, gloomy, obscure,” from P.Gmc. *dimbaz.

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

1. Math.: Independent extension in a given direction; a property of space.
   

2. Physics: → physical dimension.

Etymology (EN): From L. dimensionem (nom. dimensio), from stem of dimetri “to measure out,” from → dis- + metri “to measure.”

Etymology (PE): Vâmun, from vâ-, → dis-, + mun,
variant mân “measure” (as in Pers. terms pirâmun “perimeter,” âzmun “test, trial,”
peymân “measuring, agreement,” peymâné “a measure; a cup, bowl”), from O.Pers./Av. mā(y)- “to measure;” PIE base *me- “to measure;” cf. Skt. mati “measures,” matra- “measure;” Gk. metron “measure;” L. metrum.

Of or pertaining to → dimension.

See also: → dimension; → -al.

A technique used in physics based on the fact that the various terms in a physical equation must have identical → dimensional formulae if the equation is to be true for all consistent systems of unit. Its main uses are:

a) To test the probable correctness of an equation between physical quantities.

b) To provide a safe method of changing the units in a physical quantity.

c) To solve partially a physical probable whose direct solution cannot be achieved by normal methods.

See also: → dimensional; → analysis.

Symbolic representation of the definition of a physical quantity obtained from its units of measurement. For example, with M = mass, L = length, T = time, area = L², velocity = LT^-1, energy = ML²T^-2.
→ dimensional analysis.

See also: → dimensional; → formula.

A physical quantity or number lacking units.

See also: → dimension; → -less.

A quantity without an associated → physical dimension. Dimensionless quantities are defined as the ratio of two quantities with the same dimension. The magnitude of such quantities is independent of the system of units used. A dimensionless quantity is not always a ratio; for instance, the number of people in a room is a dimensionless quantity. Examples include the → Alfven Mach number, → Ekman number, → Froude number, → Mach number, → Prandtl number, → Rayleigh number, → Reynolds number, → Richardson number, → Rossby number, → Toomre parameter. See also → large number.

See also: → dimension<less/qot>; → quantity.

A molecule resulting from combination of two identical molecules.

See also: From → di- “two, twice, double,” + -mer a combining form denoting member of a particular group, → isomer.

An electronic component with two active terminals, an → anode and a → cathode, through which current passes in one direction (from anode to cathode) and is blocked in the opposite direction. Diodes have many uses, including conversion of → alternating current to → direct current, regulation of votage, and the decoding of audio-frequency signals from radio signals.

See also: → di- “two, twice, double,” + hodos “way.”

The fourth largest moon of Saturn and the second densest after Titan. Its diameter is 1,120 km and its orbit 377,400 km from Saturn. It is composed primarily of water ice but must have a considerable fraction of denser material like silicate rock.

See also: Discovered in 1684 by Jean-Dominique Cassini, Italian born French astronomer (1625-1712). In Gk. mythology Dione was the mother of Aphrodite (Venus) by Zeus (Jupiter).

A unit of optical measurement that expresses the refractive power of a lens or prism. In a lens or lens system, it is the reciprocal of the focal length in meters.

Etymology (EN): L. dioptra, from Gk. di-, variant of dia- “passing through, thoroughly, completely” + op- (for opsesthai “to see”) + -tra noun suffix of means.

Etymology (PE): Dioptr loanword from Fr.

An instrument used in antiquity to measure the apparent diameter of the Sun and the Moon. It was a rod with a scale, a sighting hole at one end, and a disk that could be moved along the rod to exactly obscure the Sun or Moon. The Sun was observed directly with the naked eye at sunrise or sunset in order to prevent eye damage. Aristarchus (c.310-230 B.C.), Archimedes (c. 290-212 B.C.), Hipparchus (died after 127 B.C.), and Ptolemy (c.100-170 A.D.) used the dioptra. The instrument could also serve for measurement of angles, land levelling, surveying, and construction of aqueducts and tunnels.

See also: → diopter.

Any → oxide containing two → atoms of → oxygen the → molecule.

See also: → di-; → oxide.

1. Navigation: The angular difference between the visible horizon and the true horizon. Same as → dip of the horizon.
   

2. Geodesy: The angle between the horizontal and the lines of force of the Earth’s magnetic field at any point. → magnetic dip.
   

3. Aviation: The angle between the true and apparent horizon, which depends on flight height, the Earth’s curvature, and refraction.

Etymology (EN): O.E. dyppan “to immerse,” cognate with
Ger. taufen “to baptize,” and with → deep.

Etymology (PE): Našib, → depression.

The angular difference between the → visible horizon and the → true horizon. Same as → dip of the horizon.

See also: → dip; → angle.

The angle created by the observer’s line of sight to the → apparent horizon and t he → true horizon. Neglecting the → atmospheric refraction, dip of the horizon can be expressed by θ (radians) = (2h/R)^1/2, where h is the observer’s height and R the Earth’s radius. An an example, for a height of 1.5m above the sea, and R = 6.4 x 10⁶ m, the dip angle is about 0.00068 radians, or 0.039 degrees, about 2.3 minutes of arc, quite appreciable by the eye. See also → distance to the horizon.
Same as → dip angle.

See also: → dip; → horizon.

The brightest star in the constellation → Cetus; a → red supergiant (K0 III) of visual magnitude 2.04.

Etymology (EN): Diphda, from Ar. zafda’ (ضفدع) “frog.” It is also designated as Deneb Kaitos, from zanab al-qaytusذنب القیطس “tail of Cetus.”

Etymology (PE): Mid.Pers. wazaγ, vak; Av. vazaγa- “frog,” → tadpole orbit.

Phonetics: A → vowel sound produced by a blended sequence of two separate vowels in a single syllable, where the sound begins as one vowel and moves toward another (as in loud, light, and lair).

Etymology (EN): From M.Fr. diphthongue, from L.L. diphthongus, from Gk. diphthongos “having two sounds,” from → di- “double” + phthongos “sound, voice.”

Etymology (PE): Dovâké, from do “two, → di-” + vâké, vâk, → voice.

A pathological condition of vision in which a single object appears double because the eyes are not focusing properly. Same as → double vision.

Etymology (EN): From L. diplo- “double, in pairs,” from Gk., combining form of diplos “twofold”

• -opia, from Gk. -opia, from ops “eye.”

Etymology (PE): Dobini, from do→ two + bini “vision, seeing,” from bin “to see; seer” (present stem of didan;
Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
Skt. veda “I know;” Gk. oida “I know,” idein “to see;” L. videre “to see;” PIE base *weid- “to know, to see”).

Of or relating to a → dipole.

See also: → dipole; → -ar.

A → magnetic field produced by a system possessing a net magnetic → dipole moment.

See also: → dipolar; → magnetic; → field.

1. A combination of two electrically or magnetically charged particles of opposite signs, which are separated by a very small distance.
   

2. Chemistry: A molecule containing both positively and negatively charged groups.

See also: → di- “two, twice, double,” + → pole.

A form of anistropy in the temperature of the → cosmic microwave background radiation, appearing as one hot pole and one cold pole, caused by our motion with respect to the cosmic background radiation. The temperature variations, amounting to 1 part in 1000, yield a velocity of about 600 km/sec for our Galaxy with respect to the background. → cosmic microwave background anisotropy.

See also: → dipole; → anisotropy.

One of the simplest kinds of antenna which is connected at the center to a radio-frequency feed line
for transmitting or receiving radio frequency energy. It differs from the dish antenna in that it consists of many separate antennas that collect energy by feeding all their weak individual signals into one common receiving set.

See also: → dipole; → antenna.

1. The product of the strength of either of the charges in an → electric dipole and the distance separating the two charges. It is expressed in → coulomb meters. Dipole moment is a → vector quantity. Its direction is defined as toward the positive charge. In chemistry dipole moment is a quantitative measure of polarity in a molecule;
   the unit is the → debye.
   

2. The product of the strength of either of the poles in a → magnetic dipole and the distance separating the two poles. Dipole moment is a vector quantity. Its direction is defined as toward the magnetic north pole.

See also: → dipole; → moment.

The electromagnetic radiation produced by an oscillating → electric dipole or → magnetic dipole.

See also: → dipole; → radiation.

An → isotope of → helium that consists of two → protons, without any → neutrons.
It is extremely → unstable.

See also: → di-; → proton.

Paul A. M. Dirac (1902-1984), English theoretical → physicist, one of the founders of → quantum mechanics and → quantum electrodynamics, Nobel Prize for Physics in 1933. → Dirac annihilation, → Dirac equation, → Dirac function, → Dirac constant, → Fermi-Dirac statistics .

Same as → pair annihilation.

See also: → Dirac; → annihilation.

The → Planck constant divided by 2π and denoted ħ, pronounced h-bar. Also called → reduced Planck constant.

See also: → Dirac; → constant.

The equation that describes the behavior of an → electron in a way that combines the requirements of → quantum mechanics with the requirements of → special relativity.

The Dirac equation predicted the existence of antimatter

See also: → Dirac; → equation.

A function of x defined as being zero for all values of x other than x = x₀ and having the definite integral from x = -∞ to x = +∞ equal to unity.

See also: → Dirac; → function

→ Dirac constant.

See also: → Dirac; → constant.

1a) Proceeding in a straight course or line without deviation or interruption.

1b) Math.: Varying in the same manner as another quantity, especially increasing if another quantity increases or decreasing if it decreases.

1c) Astro.: Designating west-to-east motion of a planet in the same direction as the Sun’s movement against the stars.

2. To administer; manage; supervise; to serve as a director in the performance or production of.

Etymology (EN): From L. directus “straight,” p.p. of dirigere “set straight,” from → dis- “apart” + regere “to guide;” cognate with Pers. râst, as explained below.

Etymology (PE): Râst “right, straight” (râšt in afrâštan); Mid.Pers.
râst; O.Pers. rāsta- “straight, true,” rās-
“to be right, straight, true,” rād- “to prepare;” Av. rāz-
“to direct, put in line, set,” razišta- “straightest, most correct,”
erezu- “correct, straight,” rāzayeiti “directs,” razan- “order;”
Skt. raj- “to direct, stretch,” rjuyant- “walking straight;”
Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,”
p.p. rectus “right, straight;” Ger. recht; E. right;
PIE base *reg- “right, just; to move in a straight line.”
Sarrâst, from sar here an intensive element meaning “side,” originally “head” (Mid.Pers. sar; Av. sarah- “head;” Skt. siras- “head;” Gk. kara “head,” keras “horn;” Mod.Pers. sarun “horn;” L. cerebrum “brain;” PIE *ker- “head, horn”) + râst, as above.
Râštidan, infinitive from râšt, variant râst “direct.”

A correlation between two variables such that as one variable becomes large, the other also becomes large, and vice versa. The correlation coefficient is between 0 and +1. Also called positive correlation.

See also: → direct; → correlation.

An → electric current which flows in one direction only and which is substantially constant in magnitude. Virtually all electronic and computer hardware needs direct current to function. → Alternating current can be converted to direct current by means of a power supply consisting of a → transformer.

See also: → direct; → current.

The motion of a solar system body from West to East across the sky against the background stars. It is the “normal” direction of motion within the solar system. For rotating or orbiting solar system objects it is anti-clockwise as seen from above the solar system in the direction of the North Pole. The same as → prograde motion. See also → retrograde motion.

See also: → direct; → motion.

A word or group of words representing the person or thing upon which the action of a verb is performed or toward which it is directed. In English, generally coming after the verb, without a preposition. In He saw it the pronoun it is the direct object of saw (Dictionary.com).

See also: → direct; → object.

In → graph theory, an edge where endpoints are distinguished; one is the head and the other is the tail. A directed edge is specified as an ordered pair of → vertices, u, v and is denoted by (u, v) or u→ v.

See also: → directed; → edge.

In → graph theory, a graph with → directed edges. Also calle → digraph.

See also: → directed; → graph.

In a → directed graph, a path in which all → edges are oriented in the same direction.

See also: → directed; → edge.

1. A position to which motion or another position is referred.
   
2. The act or an instance of directing.

Etymology (EN): M.E. direccioun, from M.Fr., from L. direction-, stem of directio “arranging in line, straightening,” → direct.

Etymology (PE): 1) Râstâ, from direct→ direct + -â dimension suffix; su, from Mid.Pers. sôk “side.”
2) Râštâri, noun from râštâr→ director + suffix -i.

An angle made by a given vector and a coordinate axis.
The angle between the North and a chosen direction.

See also: → direct; → angle.

A person who directs a project, a group, a production.

Etymology (EN): From → direct + -tor a suffix found in loanwords from L., forming personal agent nouns from verbs.

Etymology (PE): Râštâr, from râšt- stem of râštidan→ direct + -âr suffix of agent noun (as in parastâr “nurse”).

One of the following conditions for a → Fourier series to converge:

1. The function f(x) is defined and single valued, except possibly at a finite number of points in the interval -π, +π.
   

2. f(x) has a period of 2π.
   

3. f(x) and f’(x) are → piecewise continuous functions on -π, +π.
   

Then, the Fourier series converges to:

(a) f(x) if x is a point of continuity.

(b) (f(x + 0) + f(x - 0))/2, if x is a point of discontinuity.

See also: Named after Peter Gustav Lejeune Dirichlet (1805-1859), German mathematician who made valuable contributions to → number theory, → analysis, and → mechanics;
→ condition.

A substance, such as mud or dust, that soils someone or something (OxfordDictionaries.com)

Etymology (EN): Metathesis of M.E. drit, drytt “mud, dirt, dung,” from O.N. drit, cognate with O.E. dritan.

Etymology (PE): Cerk “dirt, filth.”

Covered or marked with an unclean substance. → dirty ice, → dirty iceball model.

See also: → dirt + suffix -y.

Interstellar ice grains with graphite, silicates, or other chemical compounds adsorbed on their surfaces.

See also: → dirty; → ice.

A model for a → cometary nucleus proposed by Fred Whipple (1950-51), according to which the nucleus is
a solid body (a few kilometers across) made up of various → ices (→ frozen water, → methane, → ammonia, → carbon dioxide, and → hydrogen cyanide) in which → dust is embedded. Dust particles are liberated when the ices vaporize as the → comet
approaches the → Sun, and they get blown away by → solar radiation pressure, often forming impressive, gently curved → dust tails.

See also: → dirty; → ice; → ball; → model.

A prefix meaning “lack of, not” (e.g. dishonest); “do the opposite of” (e.g. disallow); “apart, away” (e.g. discard); before f, dif-; before some consonants (b, d, l, m, n, r, s, v, and sometimes g and j) di-.

Etymology (EN): From O.Fr. des-, from L. dis- “apart,” from PIE *dis- “apart, asunder” (cf. O.E. te-, O.S. ti-, O.H.G. ze-, Ger. zer-). The PIE root is a secondary form of *dwis- and is thus related to L. → bis “twice.”

Etymology (PE): Vâ- prefix denoting “separation; repetition; open; reversal, opposition; 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.”

Computers: To cancel the assignment of a particular resource to a user.

See also: → dis- + → allocate.

The act of disallocating or the state of being disallocated.

See also: → dis- + → allocate.

The act or state of disapproving; a condemnatory feeling, look, or utterance.

See also: → dis-; → approval.

To withhold approval from; decline to sanction.

See also: → dis-; → approve.

The removal or transference of an electric charge, as by the conversion of chemical energy to electrical energy.
The sudden and momentary equalization of a difference of potential, as between two terminals.

Etymology (EN): From M.E. deschargen, from O.Fr. deschargier “unload,” from L.L. discarricare, from → dis- “do the opposite of” + carricare “to load a cart,” from carrus “cart.”

Etymology (PE): Vâbâr, from vâ- reversal prefix, → de- + bâr→ charge.

The quality or state of being → disciplinary.

See also: → disciplinary; → -ity.

Of, for, or constituting a → discipline. See also → interdisciplinary, → multidisciplinary, and
→ transdisciplinary.

See also: → discipline; → -ary.

1. A branch of learning or instruction.
   

2. A system of rules for behavior, methods of practice, etc.
   

Etymology (EN): M.E., from M.Fr. discipline, descepline, from L. disciplina literally “teaching, instruction,” from discipulus “pupil, student, follower,” from dis- + cipere, combining form of capere “to take,” → concept, + -ulus, → -ule.

Etymology (PE): Hâvešân, from hâveš “disciple, pupil,” Mid.Pers. hâvišt “disciple, pupil;” Av. hāvišta- “disciple, pupil,” + -ân a suffix denoting “group, order, system.”

A break in sequence or continuity of anything. → Balmer discontinuity
Math.: A point at which a function is not continuous.
Geology: A surface separating rock layers of differing properties or compositions.

Etymology (EN): M.L. discontinuitas, from discontinuus, from → dis- + continuus, → continuous.

Etymology (PE): Nâpeyvastegi, noun from nâpeyvasté “discontinuous,” from nâ- “non, un-,” → a-, + peyvasté,
→ continuous.

To find (something previously unseen or unknown) after study or search.

Etymology (EN): M.E. discoverir, descovrir, from O.Fr. descovrir “uncover, unveil, reveal,” from L.L. discooperire, from L. → dis- “opposite of” + cooperire “to cover up,” → cover.

Etymology (PE): Padvâridan “to uncover,” infinitive from padvâr, from prefix pad- “against, contrary, opposite to,” variants pat-, pâd-, → anti-,

• vâr- “to cover;” cf. Av. vār- “to cover;” Mid.Pers. (with prefix *uz-) uzvâr- “to interprete, uncover, grasp,” Pers. hozvâr(eš) “interpretation;” Khotanese (+ *ui-) gvir- “to be revealed;” Choresmian (+ *apa-) bw’rg- “to uncover;” Pers. barg “cover, clothes;” šalvâr “trousers” (Cheung, 2007).

The act or an instance of discovering. Something discovered.

See also: Verbal noun of → discover.

Divergence or disagreement, as between facts, claims, or calculations.

Etymology (EN): From L. discrepantia, from discrepantem, pr.p. of discrepare “sound differently, differ,” from → dis- “apart, off” + crepare “to rattle, crack.”

Etymology (PE): Nâhamxâni, from nâ- “non, un-,” → a-, + hamxâni “siniging together,” from ham- “together,” → com-,

• xâni, from xân stem of xândan “to sing; to read; to call,” Mid.Pers. xwân- “to resound; to call,” Av. x^van- “to sound,”
  Proto-Iranian *huan- “to call;” cf. Skt. svan-, sváranti “to sound, make a sound, sing;” L. sonus “sound,” sonare “to sound;” O.E. swinn “music, song,” PIE base *suen- “to sound” (Cheung 2007).

Apart or detached from others; separate; distinct.
Composed of distinct or discontinuous elements.

Etymology (EN): From O.Fr. discret, from L. discretus “separated, distinct,” from p.p. of discernere “to distinguish,” from → dis- “off, away”

• cernere “distinguish, separate, sift,” → crisis.

Etymology (PE): Gosasté “broken, broken off,” p.p. of gosastan “to break, split,” Mid.Pers. wisistan “to break, split,” Av. saed- “to split,” asista- “unsplit, unharmed,” Skt. chid- “to split, break, cut off,” Gk. skhizein “to split,” L. scindere “to split;” PIE stem *skei- “to split, cut.”
Jodâ “separate, distinct,” Mid.Pers. jutâk (yutâk), jut (yut).

The rapid, systematic changes in the absorption parts of the → P Cygni profiles of the ultraviolet → resonance lines (Si IV, C IV, and N V) observed in a majority of massive → hot stars. DACs are typically seen to accelerate to the → blue wing of the profile over a few days, becoming narrower as they approach the → terminal velocity.

See also: → discrete; → absorption; → component.

Data that can only take a small set of particular values, usually whole numbers. For instance, number of children in a family cannot be 2.4, since
parents cannot actually have 2.4 children. → continuous data.

See also: → discrete; → data.

A localized source on the celestial sphere that can be observationally separated at radio wavelengths from its background emission.

See also: → discrete; → radio;
→ source.

A spectrum in which the component wavelengths (and wave numbers and frequencies) constitute a discrete sequence of values (finite or infinite in number) rather than a continuum of values.

See also: → discrete; → spectrum.

A transition between two quantum-mechanical energy levels. See also → discrete spectrum.

See also: → discrete; → transition.

A quantity that has certain magnitudes and does not represent a → continuous variable.

See also: → discrete; → value.

A variable which has only → discrete values and has no in-between values.

See also: → discrete; → variable.

A → quantum walk involving a probabilistic → operator that changes the direction while leaving the position fixed, and a shift operator that changes the position. Discrete-time quantum walk was introduced by J. Watrous (2001, Journal of Computer and System Sciences 62, 376)

See also: → discrete; → time; → quantum; → walk.

To make a distinction in favor of or against a person or thing on the basis of the group, class, or category to which the person or thing belongs rather than according to actual merit (Dictionary.com).

Etymology (EN): From L. discriminatus, p.p. of discriminare “to divide, separate,” from discrimen “a space between, division, separation, difference,” from discernere “to separate, divide; distinguish, perceive,” from → dis- “off, away” + cernere “distinguish, separate, sift;” ultimately from PIE root *krei- “to sieve,” → critique.

Etymology (PE): Vâkarbidan, from prefix vâ-, → dis-,

• karb-, related to stem qarb- in qarbâl, qalbur, gerbâl “sieve,” probably related to PIE root *krei- “to sieve,” as above.

1. An act or instance of discriminating, or of making a distinction.
   

   2. Unfair treatment of a person, racial group, minority, etc; action based on prejudice (Dictionary.com).

See also: Verbal noun of → discriminate.

Characterized by or showing prejudicial treatment, especially as an indication of bias related to age, color, national origin, religion, sex, etc. (Dictionary.com).

See also: From → discriminate + adjective and noun suffix -ory.

To → consider or → examine by → argument, → comment, etc.; talk over or write about, especially to explore solutions; → debate (Dictionary.com).

Etymology (EN): M.E., from Anglo-Fr. discusser, from L. discussus “struck asunder, shaken, scattered,” p.p. of discutere to break up, “strike asunder,” from → dis- “apart,” + quatere “to shake, strike.”

Etymology (PE): Bahsidan, infinitive from bahs, from Ar. baHS (بحث) “investigating, examining, controversy, debate, dispute.”

An act or instance of discussing; → consideration or → examination by → argument, → comment, etc., especially to explore solutions; informal debate (Dictionary.com).

See also: Verbal noun from → discuss.

A large parabolic structure that collects radio waves and focuses them on a detector by means of a secondary reflector. A similar device used as antenna for radar transmitting.

Etymology (EN): O.E. disc “plate, bowl, platter,” from L. discus “dish, platter, quoit,” from Gk. diskos “disk, platter.”

Etymology (PE): Jâm “cup, chalice, goblet, bowl,” Mid.Pers. jâm “vessel, goblet; glass,” Av. yama- “glass, glass vessel,” yāmô.pacika- “baked glass;” related to Skt. camasa- “a vessel used at sacrifices for drinking Soma, kind of flat dish or cup?”

To remove infection the use of appropriate means.

See also: → dis-; → infect.

Any agent, such as a chemical, heat, or radiation, that destroys or inhibits the growth of disease carrying microorganisms.

See also: → disinfect; → -ant.

The breaking up of a body into fragments. For example, that of an
unstable nuclei either spontaneously or as a result of bombardment by fast-moving particles, or the breaking up of a comet.

Etymology (EN): Disintegration, verbal noun of disintegrate, from → dis- + → integration.

Etymology (PE): Vâpâši, forupâši, from vâ-, frou-→ de-

• pâši, noun of pâšidan→ disperse.

Any of the components in a logical → disjunction.

See also: → disjunction.

A → proposition of the form “A or B” (A ∨ B), where A and B are themselves propositions. A disjunction is → true when one or the other of its components (called → disjuncts) is true, and false otherwise.

See also: → dis-; → junction.

1. General: Any thin, flat, circular plate or object.
   
2. Math.: The domain bounded by a circle.
   
3. Astro.: A thin, flattened material structure encompassing a central object, such as a star or a galactic nuclear region. → accretion disk;
   → circumnuclear disk; → circumstellar disk; → galactic disk.
   
   Computers: Any of several types of material support on which data can be encoded.

Etymology (EN): From L. discus “quoit, discus, disk,” from Gk. diskos, from dikein “to throw.” → dish.

Etymology (PE): Gerdé, from Mid.Pers. girdag “disk, round,” from gird/girt “round, all around,” Proto-Iranian *gart- “to twist, to wreathe,” cf. Skt krt “to twist threads, spin; to wind; to surround;” kata-
“a twist of straw,” Pali kata- “ring, bracelet,” Gk. kartalos “a kind of basket,” kyrtos “curved;” disk loanword from Fr.

An accretion process involving an → accretion disk.

See also: → disk; → accretion.

A galaxy consisting of a thin disk of stars and → interstellar matter which may include → spiral arms and → bar.

See also: → disk; → galaxy.

1. General: The process by which an → accretion disk cools, causing it to fragment into self-gravitating → clumps.
   

2. A model of → planet formation in which a region of a → circumstellar disk becomes dense and cool enough to be unstable to → gravitational collapse, resulting in the formation of a gaseous → protoplanet. Disk instability
   has been put forward as a mechanism for forming massive → giant planets on short time-scales (1 Ky-1 My).
   

3. In models of galactic structure, a process that is thought to cause the formation of → pseudo-bulges.
   

4. → disk instability model (DIM).

See also: → disk; → instability.

A model describing → dwarf novae and → Soft X-ray Transient (SXT)s. Accordingly, these objects are triggered by an → accretion disk instability due to an abrupt change in opacities (→ opacity) at → temperatures at which hydrogen is partially ionized. All versions of the DIM have this ingredient. They differ in assumptions about → viscosity, and about what happens at the inner and outer disk radii. Basically, during → quiescence, material accumulates in the accretion disk until a critical point is reached. The disk then becomes unstable and is dumped onto the → compact object, releasing a burst of → X-rays. However, the greater duration of SXT bursts (months) and the time interval between bursts (decades) cannot be accounted for by the standard disk instability model used for dwarf novae, and additional factors such as X-ray illumination and irradiation of the accretion disk are required for the model to match the observed properties of SXTs (J-P Lasota and J-M Hameury, 1995).

See also: → disk; → instability;
→ model.

In star formation models involving magnetized accretion, a process whereby the stellar rotational → angular velocity becomes equal to the → Keplerian angular velocity of the → accretion disk. This happens at the → corotation radius. Disk locking is believed to be responsible for efficient loss of stellar → angular momentum during the → pre-main sequence contraction of → T Tauri stars. These stars are expected to spin up by a factor of about 3 due to contraction after being magnetically disconnected from the → circumstellar disk. However, observations show that a large fraction of pre-main sequence stars evolve at nearly constant angular velocity through the first 4 Myr. This process results from → magnetic braking. The idea of magnetic disk locking originated with the theory developed by Ghosh & Lamb (1979, ApJ 232, 259) for → neutron stars.

Etymology (EN): M.E., from O.E. lucan “to lock, to close,” from loc “bolt, fastening, enclosure;” cf. M.L.G. lok, O.H.G. loh, O.N. lok “a cover, lid,” Goth. -luk in usluk “opening,” Ger. Loch “opening, hole,” Du. luck “shutter.”

Etymology (PE): Girkard, from gir “hold, block,” from gereftan “to seize, hold, take,” → eclipse, + kard noun from kardan “to do,” → work.

A logical division of a hard disk that is treated as a separate unit by operating systems and file systems.

See also: → disk; → partition.

Of a spiral galaxy, those stars that lie in a flattened disk and move in nearly circular orbits around its centre. They are Population I stars of all ages up to the age of the disk, but in general are younger than stars in → halo population.

Etymology (EN): → disk;
population, from L.L. populationem “a people, multitude,” from populatio, from populare “to inhabit,” from populus “people,” related to plebes “the common people,” cf. Gk. plethos “people, multitude, great number,” from PIE base *pel- “to be full;” Mod.Pers. por “full,” O.Pers. paru- “much, many,” Av. pouru- “much, many,” pərəna- “full,” par- “to fill,” Skt. puru-, Gk. polus, O.E. full “completely, full,” from P.Gmc. *fullaz, O.H.G. fol, Ger. voll, Goth. full.

Etymology (PE): Like Gk., Pers. uses the concepts of “multitude, many, full” to denote “people, group, herd, flock”. The following examples are all terms derived from O.Pers. paru- “much, many,” Av. par- “to fill,” pouru- “much, many,” pərəna- “full” (Mod.Pers. por “full”): literary Pers. bâré “herd, flock,” parré “a rank or file of soldiers, a circular disposition of troops,” Lori, Qâyeni bor “group, tribe, herd,” Torbat-Heydariyeyi, Qomi borr “heap, bundle, group,” Qomi borreh “group, assemblage of people,” Pashtu parrak “flock, herd,” Urdu para “flock, herd,” Lârestâni baila “group, tribe,” Tabari balik “herd, flock.” With this introduction,
porineš “population,” verbal noun of porinidan “to populate,” infinitive of porin “populous,” from por “mutitude, many, full”

• -in attribution suffix.

Computers: The specific amount of disk space that a user or service is allowed to use.

See also: → disk; → quota.

The process whereby a → galaxy evolves from a disturbed to an ordered system, as it develops into a → rotation dominated → settled disk.

See also: → disk; → settling.

A star that lies within the → galactic disk of a → spiral galaxy.
Stars belonging to the → thin disk, such as the Sun or Alpha Centauri, lie at a typical distance of about 1,000 → light-years from the galactic midplane. There are also → thick disk stars, such as Lalande 21185, that lie at an average distance of about 3,500 light-years from the midplane.

See also: → disk; → star.

In models of magnetized → accretion disks, the process whereby the disk is disrupted at a radius where the → magnetic pressure overcomes the → ram pressure of the accreted material. This occurs at a distance typically 3-7 stellar radii, below the → corotation radius.

See also: → disk; → truncation.

In → magnetocentrifugal models of → protostars, the wind arising from a significant range of radii in the → accretion disk. The contribution from innermost parts of the disk is dealt with by the → X-wind model. (Königl A. and Pudritz R. E., 2000, In Protostars and Planets IV,
V. Mannings, et al. (eds.), Tucson: Univ. Arizona Press, p. 759).

See also: → disk; → wind.

A → galaxy bulge that is flatter than a → classical bulge. Such bulges
might be difficult to see in very inclined galaxies. They may contain sub-structures such as nuclear → bars, → spiral arms, or → rings. They usually show signs of → dust obscuration, younger → stellar populations,
or ongoing → star formation. These systems seem to form mostly through disk instabilities (→ disk instability), such as bars, in a relatively slow, continuous and smooth process. Essentially, such instabilities induce a redistribution of → angular momentum along the galaxy, and, as a result, mostly gas but also stars are driven to the disk center. Also called → pseudo-bulge (Kormendy & Kennicutt, 2004, ARA&A 42, 603; Fisher & Drory, 2010, ApJ 716, 942).

See also: → -disk; → like; → bulge.

A lack of → order. See → entropy.

Etymology (EN): → dis-; → order.

Etymology (PE): Birâyegi, from birâyé, → disordered, + -(g)i noun suffix.

Lacking → order.

Etymology (EN): → disorder + -ed.

Etymology (PE): Birâyé, from bi- “without,” → a-,

• râyé, → order.

1. To spread or distribute from a fixed or constant source.
   

2. To become dispersed.

Etymology (EN): M.E., from M.Fr. disperser “scatter,” from L. dispersus, p.p. of dispergere “to scatter,” from → dis- “apart” + spargere “to scatter,” from PIE base *(s)pregh- “to scatter;” cf. Av. spareg- “to germinate, shoot, sprout,” fra-sparəγa- “shoot, sprout,”
Skt. parjanya- “rain, rain god,” Lith. spurgas “sprout.”

Etymology (PE): Pâšidan “to scatter, sprinkle,” az ham pâšidan “to scatter on all sides;” cf. Gazi pâšn-/pâšnâ “to scatter, spread,” Lor. perxa “sprinkling;” Av. paršat.gauu- “having a speckled cow;”
Skt. prs- “to sprinkle,” parsati “sprinkles;” Toch. pärs- “to sprinkle;” Lith. purškiu “I spray;” PIE roor *pers- “to spray, sprinkle.”

1. The resolution of white light into its component wavelengths, either by → refraction or by → diffraction.
   Dispersion is actually an effect in which radiations having → different wavelengths travel at different speeds in the medium. Since the → angle of refraction of each radiation vary as a function of its → wavelength, the component waves deviate from each other.
   

2. The rate of change of → refractive index with wavelength: dn/dλ. → Cauchy’s equation shows that dispersion varies approximately as the inverse cube of the wavelength.
   

3. The selective → retardation of → radio waves when they propagate through a → plasma. As a result, higher → frequency waves from a → pulsar will arrive at the Earth before the lower frequencies due to the presence of → intervening → ionized gas in the → interstellar medium.
   → dispersion measure.
   

4. Statistics: The spread of values of a → variable around the → mean or → median of a → distribution.

See also: Verbal noun of → disperse.

A graph displaying the variation of the → refractive index of a substance against the wavelength of the electromagnetic wave passing through the substance.

See also: → dispersion; → curve.

An equation representing the variation of → refractive index as a function of → wavelength; for example → Cauchy’s equation and → Sellmeier’s equation.

See also: → dispersion; → equation.

A parameter used in radio astronomy which describes the amount of dispersion in a radio signal due to its passage through an intervening plasma. It is proportional to the product of the interstellar electron density and the distance to the source.

See also: → dispersion; → measure.

An equation that describes how the → angular frequency, ω, of a wave depends on its → wave number, k. For the simplest of waves, where the speed of propagation, c,
is a constant, ω(k) = ck. If the → phase velocity depends on k, that is for a dispersive medium, the function ω(k) is nonlinear.

See also: → dispersion; → relation.

Tending or serving to disperse.

See also: From → disperse + → -ive.

The reciprocal of the → dispersive power.

See also: → dispersive; → index.

A medium in which the → phase velocity is a function of → wave number (→ frequency).

See also: → dispersive; → medium.

A measure of the ability of a medium to separate different colors of light. It is defined by: (n₂ - n₁)/(n - 1), where  n₁ and n₂ are refractive indices at two specified widely differing wavelengths, and n is the → index of refraction for the average of these wavelengths.

See also: → dispersive; → power.

Physics: A vector quantity that specifies the change of position of a body or particle from the mean position or position of rest.
Geology: The offset of rocks caused by movement along a fault.

Etymology (EN): From displace, from → dis- + place + -ment.

Etymology (PE): Jâ bé jâyi, noun of jâ bé jâ literally “place to place,” from jâ “place,” from Mid.Pers. giyag “place,” O.Pers. ā-vahana- “place, village,” Av. vah- “to dwell, stay,” vanhaiti “he dwells, stays,” Skt. vásati “he dwells,” Gk. aesa (nukta) “to pass (the night),”
Ossetic wat “room; bed; place,” Tokharian B wäs- “to stay, wait;” PIE base ues- “to stay, live, spend the night.”

In electromagnetism, a quantity which is not a real current (movement of charge), but has the units of current and has an associated magnetic field. The physical meaning of this displacement current is that a changing electric field makes a changing magnetic field.

See also: → displacement; → current.

1a) To show; make visible.

1b) Of computers, to show the output data on a screen.

2a) The act of displaying.

2b) Of computers, a visual representation of information.

Etymology (EN): From O.Fr. despleier “to unfold, spread out,” from L. displicare “to scatter,” from → dis- “un-, apart” + plicare “to fold, twist,” from PIE base *plek- “to plait, twist” (cf. Gk. plekein “to plait,” L. plectere “to plait, braid, intertwine,” Av. frašna- “helmet” (in ərəzatô.frašna- “having a silver helmet” (epithet of Mithra), Skt. praśna- “turban,” O.H.G. flehtan “to plait,” Russ. plesti “to plait”).

Etymology (PE): Nemudan, nemâyândan “to show, display,” from Mid.Pers. nimūdan, present stem nimây- “to show,” from O.Pers./Av. ni- “down; into,” → ni-, + māy- “to measure,” cf. Skt. mati “measures,” matra- “measure,” PIE base *me- “to measure;” cf. Gk. metron “measure,” L. metrum.

1. To incur the dissatisfaction, dislike, or disapproval of; offend; annoy.
   

2. To be unpleasant; cause displeasure (Dictionary.com).

See also: → dis-; → please.

Dissatisfaction, disapproval, or annoyance.

See also: → dis-; → pleasure.

An act or instance of disqualifying; the state of being disqualified.

See also: → disqualify; → -tion.

(of a person) declared ineligible for an office, activity, or competition because of an offence or infringement (OxfordDictionaries.com).

See also: → dis-; → qualified.

To deprive of qualification or fitness; render unfit; incapacitate (Dictionary.com).

See also: → dis-; → qualify.

Forcible division into pieces of an object. → tidal disruption.

Etymology (EN): From L. disruptio “a breaking asunder,” noun of action from p.p. stem of disrumpere “break apart, split, shatter, break to pieces,” from → dis- “apart” + rumpere “to break.”

Etymology (PE): Gosixt, past stem of gosixtan “to tear away, to beark off.”

1. To cut apart (an animal body, plant, etc.) to examine the structure, relation of parts, or the like.
   

2. To examine minutely part by part; analyze: to dissect an idea (Dictionary.com).

Etymology (EN): L. dissectus, p.p. of dissecare “to cut to pieces,” from
dissecare “cut in pieces,” from → dis- “apart” + secare “to cut,” → section.

Etymology (PE): Vâsekanjidan, from vâ- “apart,” → dis-, + sekanjidan “to cut to pices,” cognate with šekastan “to break,” → section.

The act of dissecting.

See also: → dissect; → -tion.

1. To scatter in various directions. To spend or use wastefully.
   

2. To cause to lose (energy, such as heat) irreversibly. See also: → dissipation, → dissipative.

Etymology (EN): From L. dissipatus, p.p. of dissipare “to disperse, squander,” from → dis- “apart” + supare “to throw, scatter.”

Etymology (PE): Eftâl, eftâleš, from eftâlidan “to disperse; to tear; to break,” ultimately from Proto-Ir. *abi-tard-, from *tard- “to pierce, split;” cf. Skt. tard- “to split, pierce, open;” Lith. trandéti “to be eaten by moths or worms;” PIE base *terd-
“to pierce” (Cheung 2007).

The loss of energy over time by a → dynamical system, typically due to the action of → friction or
→ turbulence. The lost energy is converted into heat, raising the temperature of the system. See also: → Ohmic dissipation. → viscous dissipation.

See also: Noun form of → dissipate.

Relating to → dissipation.

See also: → dissipate; → -ive.

A → dynamical system which undergoes energy → dissipation. Such a system gives rise to → irreversible processes, associated with a time-asymmetric evolution of observable quantities.

See also: → dissipative; → system.

General: To separate from association of any kind.

See also: Verbal form of → dissociation.

General: An act or instance of dissociating; the state of being dissociated.
Chemistry: Separation of a molecule into two or more fragments (atoms, ions, radicals) by the absorption of electromagnetic radiation or the action of collisional phenomena.

Etymology (EN): From → dis- + (as)sociation, → association.

Energy required to dissociate a molecule. → dissociate.

See also: → dissociation; → energy.

Of, relating to, or tending to produce → dissociation.

See also: Adj. of → dissociate.

A process where a positive molecular ion recombines with an electron, and as a result it dissociates into two neutral products. For example, AB⁺ + e^-→ A + B, where e^- is an electron, AB⁺ is a diatomic or polyatomic molecular ion, and A and B are the neutral fragmentation products. Dissociative recombination is the dominant recombination process in planetary ionospheres and interstellar clouds.

See also: → dissociative; → recombination.

Chemistry: The process by which a solid, gas, or liquid is dispersed homogeneously in a gas, solid, or a liquid.

See also: Verbal noun of → dissolve.

To make a solution of, as by mixing with a liquid; pass into solution.

Etymology (EN): From L. dissolvere “to loosen up, break apart,” from → dis- “apart” + solvere “to loose, loosen, untie,” from PIE *se-lu-, from reflexive pronoun *swe- + base *leu- “to loosen, divide, cut apart” (cf. Gk. lyein “to loosen, release, untie,” Skt. lunati “cuts, cuts off,” lavitram “sickle,” O.E. leosan “to lose,” leas “loose.”

Etymology (PE): Vâluyidan, infinitive from stem vâlu(y)-, from vâ-→ de- + lu, variant of Mod.Pers. las “loose,” lâ “slit, cut,” luš “torn,” lok “torn, piece,” lâc “open, wide open” (→ analysis), from PIE
*leu- “to loosen, divide, cut apart.”

Absence or lack of symmetry

Etymology (EN): From → dis- + → symmetry.

Etymology (PE): Nâhmâmuni, from nâ- “non, un-,” → a- + hmâmuni→ symmetry.

Geology: Impact ejecta found at distances more than 5 crater radii from the rim of the source crater.

Etymology (EN): Distal, from dist(ant), → distance +
→ -al; → ejecta.

Etymology (PE): Ešânâk, → ejecta;
dur, → distance.

1. The separation/length in space/time between two things/events.
   

   2. The state of being apart in space or time.
      

2. In cosmology four main distance definitions are used: → luminosity distance, → angular diameter distance, → comoving distance, and → light-travel distance.

In a → flat Universe these four approaches give the same result for the present epoch for distances below 100 Mpc. In a non-flat Universe with the → Robertson-Walker metric they give different but related values.

See also:
→ apparent distance, → cosmic distance scale, → cosmological distance, → distance function, → distance modulus, → distance to the horizon, → focal distance, → Hubble distance, → perihelion distance, → polar distance, → proper distance, → redshift-distance relation, → velocity-distance relation, → zenith distance.

Etymology (EN): M.E., from O.Fr., from L. distantia “a standing apart,” from distantem (nominative distans) “standing apart, separate, distant,” pr.p. of distare “to stand apart,” from → dis- “apart, off” + stare “to stand,” (cf. Mod.Pers. istâdan “to stand,” O.Pers./Av. sta- “to stand, stand still; set,” Skt. sthâ- “to stand,” Gk. histemi “put, place, weigh,” stasis “a standing still”).

Etymology (PE): Apest, literally “standing apart,” from apa- prefix denoting “separation, away, off,” → dis-, + est variant of ist, present stem of istâdan, to stand," as above;" cf.
Choresmian bst “to stand apart,” from *apa- + st- “to stand,” → stand.
Durâ, duri, noun from dur “far, distant, remote,” Mid.Pers. dur, O.Pers. dūra- “far (in time or space),” Av. dūra-, from dav- “to move away,” cf. Skt. dūrá- “distant, far.”

Same as → metric.

See also: → distance + → function.

The difference between the → apparent magnitude (m) of a star or galaxy and its → absolute magnitude (M). It is given by m - M = 5 log d - 5, where d is the distance in → parsecs. For an object that is 10 pc away, the distance modulus is zero.

See also: → distance; → modulus.

The distance separating an observer and the → apparent horizon of the place. Neglecting the → atmospheric refraction, it is given by: d = (2Rh)^1/2, where R is the radius of the Earth and h is the observer’s height. This can be approximated to:
d (km) = 3.57(h)^1/2 for a typical value of R = 6378 km. The atmospheric refraction, however, makes the thing more complex, depending on the temperature and density variations along the line of sight. Generally, refraction pushes the apparent horizon about 10% farther.

See also: → distance; → horizon.

1. Readily distinguishable from all others.
   

   2. Clearly defined; unquestionable (TheFreeDictionary.com).

Etymology (EN): Past-participle adjective from obsolete distincten “to distinguish one thing from another; make distinct,” from O.Fr. distincter, from L. distinctus, p.p. of distinguere “to separate between, mark off.”

Etymology (PE): Bažnâ, literally “high,” from Kurd. bažn “height, stature,” variants baž, baš “height,” bašn, → stature, faš, baš “a horse’s mane.”

1. The act or an instance of distinguishing or differentiating.
   

   2. A distinguishing feature.
      

   3. The state of being different or distinguishable.
      

   4. Special honor, recognition, or fame (The FreeDictionary.com).

See also: → distinct; → -tion.

Serving to identify; distinguishing or characteristic.

See also: → distinct; → -ive.

In a clear and noticeable way; clearly.

See also: → distinct; → -ly.

The state or quality of being distinct.

See also: → distinct; → -ness.

1. To perceive as being different or distinct.
   

   2. To demonstrate or describe as being different or distinct.

Etymology (EN): ME. distingwen, from O.Fr. distinguer, from L. distinguere “to separate between, divide, mark off,” perhaps literally “separate by pricking,” from assimilated form of → dis- “apart”

• -stinguere “to prick.”

Etymology (PE): Bažnâyidan, verb from bažnâ, → distinct.

1. Characterized by excellence or distinction; eminent.
   

   2. Dignified in conduct or appearance (TheFreeDictionary.com).

See also: Past participle of → distinguish.

To twist awry or out of shape; make crooked or deformed (Dictionary.com).

Etymology (EN): From L. distortus, p.p. of distorquere “to distort,” from → dis-, + torquere “to twist.”

Etymology (PE): Cowlé “distorted, crooked, bent,” variants [Mo’in] kowlé, kal, kil, Lori cowel, Laki hoval, hol, Malâyeri caval, hol, Tabari, Aftari val, Mid.Pers. xwahl “bent, crooked;” PIE base *klei- “to lean, incline” from which is also derived
Gk. klinein “to cause to slope, slant, incline,” L. clinare “to bend,” → declination.

1. Extent to which a system, optical, acoustic, or electronic, fails to reproduce accurately at its output the characteristics of the input.
   

2. Optics: An optical imperfection caused by a → lens or → system of lenses which results in → magnification differences between different points on the → image. The points on the → object are misplaced in the image relative to the → center of the → field. See also → barrel distortion;
   → pincushion distortion.

See also: Verbal noun of → distort.

1. (v.tr.) To divide and dispense in portions; to disperse through a space or over an area.
   

2. (v.intr.) Math.: To be → distributive. → distribution.

Etymology (EN): Distribute, from L. distributus p.p. of distribute “deal out in portions,” from → dis- + tribuere “to pay, assign, allot,” from tribus “tribe.”

Etymology (PE): Vâbâžidan, infinitive of vâbâž, from vâ-→ dis- + bâž “tribute, toll, impost,” from Mid.Pers.
bâj, bâž “tribute, tax,” baxtan “to distribute,” baxt “luck, fate,” O.Pers. bāji- “tribute, tax,” Av. bag- “to distribute, divide, allot,” cf. Skt. bhaj- “to share, distribute, apportion,” Gk. phagein “to eat (to have a share of food)”; PIE base *bhag- “to share out, apportion;” → division.

An act or instance of distributing; the state or manner of being distributed; something that is distributed. → binomial distribution, → Bose-Einstein distribution, → brightness distribution, → chi-square distribution, → cumulative distribution function, → distribution function, → Gaussian distribution, → Gibbs canonical distribution, → lognormal distribution, → Maxwell-Boltzmann distribution, → normal distribution, → Poisson distribution, → power-law distribution, → probability distribution, → spectral energy distribution.

See also: Verbal noun of → distribute

A function that gives the relative frequency with which the value of a statistical variable may be expected to lie within any specified interval. For example, the Maxwellian distribution of velocities gives the number of particles, in different velocity intervals, in a unit volume.

See also: → distribution; → function.

Characterized by or relating to → distribution. → distributive law.

See also: → distibution; → -ive.

Math.: In multiplication, the principle that permits the multiplier to be applied separately to each term in the multiplicand: x(y + z) = xy + xz. → associative law; → commutative law.

See also: → distributive; → law.

The state or quality of being distributive.

See also: Noun of → distributive.

1. To interfere with; interrupt; hinder.
   

2. To interfere with the arrangement, order, or harmony of; disarrange (Dictionary.com).

Etymology (EN): M.E., from O.Fr. destorber, from L. disturbare “to throw into disorder,” from → dis- “completely” + turbare “to disorder, disturb,” from turba “turmoil,” → turbid.

Etymology (PE): Parišândan, infinitive from parišân “dispersed, scattered,” also parišidan “to disperse, get disturbed,” ultimately from Proto-Ir. *parišan- literally “disperse around,” from *pari-, Pers. par-, pirâ-, → peri-, + *šan- “to shake;” cf. afšândan, → volcano; Choresmian šny- “to tremble;” Zazaki šânâyiš/šânen- “to shake, scatter;” Kurd râšândin “to spread;” Tabari šanne “he shakes” (Cheung 2007).

The act of disturbing. The state of being disturbed.

See also: Verbal noun of → disturb.

Having a period of, occurring in, or related to a → day.

Etymology (EN): L.L. diurnalis, from V.L. diurnum “day” (Fr. jour), from L. diurnus “daily,” from dies “day” + -urnus, an adj. suffix denoting time. Dies “day” from PIE base *dyeu- “to shine;”
cf. Gk. delos “clear;” L. deus; Skt. deva “god;”
Mod.Pers. div “devil, demon;” O.Pers. daiva- “evil god, demon;” Av. daēva- “evil spirit, false god;” Gk. Zeus “supreme god.”

Etymology (PE): Ruzâné, from ruz→ day + -âné a suffix forming adverbs and adjectives.

The aberration of a star’s position due to the rotation of the Earth. Its value depends on the latitude of the observer, and is only 0’’.32 in the case of an observer at the equator, where the rotational velocity is greatest.

See also: → diurnal; → aberration.

The apparent path of an object in the sky during one day, due to Earth’s rotation.

See also: → diurnal; → circle.

Daily geometrical libration of the Moon arising from the fact that observers at different points on the Earth see the Moon from slightly different angles. As the Moon rises in the east, you are positioned on one side of our planet, and by the time it sets in the west. Earth’s rotation has carried you to the other side. This change in position produces a slight → parallax effect that adds about another 1° of libration in longitude. Two other geometrical libration are → libration in longitude and → libration in latitude. See also → physical libration.

See also: → diurnal; → libration.

The daily apparent motion of all celestial objects, due to Earth’s rotation.

See also: → diurnal; → motion.

The apparent difference between the position of a celestial object measured from the Earth’s surface and the position that would be recorded by a hypothetical observer at the center of the Earth. Same as → geocentric parallax.

See also: → diurnal; → parallax.

1a) An act or instance of diving.

1b) A jump or plunge into water, especially in a prescribed way from a diving board.

2a) To go below the surface of the water, as a submarine.

2b) To → plunge into water, especially headfirst.

2c) Aeronautics: (of an airplane) to descend rapidly (Dictionary.com).

Etymology (EN): M.E. diven “to dive, dip,” O.E. dufan (strong verb) “to dive, duck, sink” and dyfan “to dip, submerge” (weak, transitive), related to → deep.

Etymology (PE): Quté is possibly Pers., since Farhang-e Asadai records it with “t” and not Ar. “tayn” (Dehxoda), although its etymology is not established. We propose ultimately from Proto-Ir. *ui-pat-, from *pat- “to fall, to fly,” to which is related Pers. oft-, oftâdan “to → fall;” cf. Pashto. qupah “a dip, a dive, a plunge.”

(v.intr.) To move, or extend in different directions from a common point.
Math.: Of a sequence or series, to have no unique limit.

Etymology (EN): M.L. divergere, from → di- “apart,” variant of → dis- + vergere “to bend, turn, incline,” from PIE *werg- “to turn,” from base *wer- “to turn, bend” (cf. L. vertere “to turn,” Av. varət- “to turn,” Mod.Pers. gard, gardidan “to turn,” Skt. vartate “turns round, rolls,” Gk. rhatane “stirrer, ladle,” Ger. werden, O.E. weorðan “to become.”

Etymology (PE): Verâyidan, from vâ- “apart,” → de-, + gerâyidan “to incline toward; to intend; to make for,” infinitive of gerâ, the etymology of which is not clear. Gerâ may be a variant of Mod.Pers. kil “bent, inclined” (k/g and l/r interchanges),
from PIE base *klei- “to lean, incline,” cognate with L. clinare “to bend” (E. declination, inclination, etc.), Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” P.Gmc. *khlinen (Ger. lehnen, E. lean).

1. The act, fact, or amount of diverging.
   

2. Math.: A differential → operator that, for a three-dimensional vector function F(x,y,z), is the sum of the → scalar products of the unit vectors and the partial derivatives in each of the three component directions. It is written as div F or ∇ . F = (∂F/∂x)i + (∂F/∂y)j + (∂F/∂z)k, ∇ being the → del operator or → nabla. The divergence of a vector field is a → scalar field. In physics, this operator is used to derive the amount of flux per unit volume leaving an infinitesimal element of volume at a point in a vector field. For example, in an electrostatic field, the divergence of the field is zero unless the volume element contains an electric charge.

Etymology (EN): From diverge, → diverge, + -ence a noun suffix.

Etymology (PE): Vâgerâyi, from vâgerâ stem of vâgerâyidan,
→ diverge, + noun suffix -i.

Same as → Gauss’s theorem.

See also: → divergence; → theorem.

Relating to or causing divergence. Gowing away in different directions from a common point or path.
Math.: Failing to approach a limit.

Etymology (EN): M.L. divergent-, stem of divergens pr.p. of divergere, from → di- “apart,” variant of → dis- + vergere “to bend, turn, incline,” from PIE *werg- “to turn,” from base *wer- “to turn, bend” (cf. L. vertere “to turn,” Av. var ət- “to turn,” Mod.Pers. gard, gardidan “to turn,” Skt. vartate “turns round, rolls,” Gk. rhatane “stirrer, ladle,” Ger. werden, O.E. weorðan “to become.”

Etymology (PE): Vâgerâ, agent noun from vâgerâyidan,
→ diverge + noun suffix -i.

A lens which causes a parallel beam of light passing through it to diverge or spread out; concave lens. Same as diverging lens and → negative lens.

See also: → divergent; → lens.

1. Differing from one another.
   

2. Composed of various kinds or forms; multiform.

Etymology (EN): M.E., from L. diversus, p.p. of divertere “to divert,” from → di- + vert, from vertere “to turn,”
→ convert.

Etymology (PE): Gunâgun, literally “of different kinds, sorts, species,” from gun “kind, species, sort.”

The state or fact of being diverse; difference; unlikeness.

See also: → diverse; → -ity.

A number which is to be divided by another number (→ divisor). Example: 36 in the expression 36 : 9 = 4.

Etymology (EN): From L. dividendum “thing to be divided,” → division.

Etymology (PE): Bâxši, from bâxš, → division.

1. The act or process of dividing; state of being divided.
   Math.: An operation in which a dividend is divided by a divisor to give a quotient and a remainder.
   
2. Something that marks a division; a separation.

Etymology (EN): From O.Fr. division, from L. divisionem (nom. divisio), from divid-, stem of dividere “to cleave, distribute,” from → dis- “apart” + -videre “to separate,” from PIE base *widh- “to separate.”

Etymology (PE): 1) Baxš “portion, part, division,” baxšidan “to divide, distribute, grant;” Mod./Mid.Pers. baxt “fortune, fate,” baxtan, baxšidan “to distribute, divide,” bâq “garden,” initially “piece or patch of land,” baq “god, lord;” Av. bag- “to attribute, allot, distribute,” baxš- “to apportion, divide, give to,”
baxta- “what is allotted (luck, fortune),” baxədra- “part, portion,” baγa- “master, god;” O.Pers. bāji- “tribute, tax;” cf. Skt. bhaj- “to share, divide, distribute, apportion,” bhájati “divides,” bhakta- “allotted; occupied with; a share; food or a meal, time of eating?” pitu-bháj- “enjoying food;” Gk. phagein“to eat (to have a share of food)”; PIE base *bhag- “to share out, apportion.”

2. Šekâf “a fissure, crack, slit,” šekâftan “to divide into two long strips, split, break,” Mid.Pers. škâftan.

A symbol placed between two quantities (dividend and the divisor) to indicate the division of the first by the second. The division sign is written as a horizontal line with dot above and dot below, ÷ (→ obelus), or a slash or horizontal line.

See also: → division; → sign.

A number by which another number, the → dividend, is divided.

Etymology (EN): → division; → -or.

Etymology (PE): Baxšyâb, literally “division finder, ~ obtainer,” from baxš, → division, + yâb, → finder.

1a) The legal dissolution of a marriage by a court or other competent body.

1b) A legal decree dissolving a marriage.

2. Legally dissolve one’s marriage with someone (OxfordDictionaries.com).

Etymology (EN): M.E., from O.Fr. divorce from L. divortium “separation, dissolution of marriage,” from divertere “to turn in different directions, to separate,” from di- blended with → de- “apart,” + vertere “to turn,” → version.

Etymology (PE): Vosin, from Sangesari vovsin “divorce,” related to Mid.Pers. visastan “to break,” Mod.Pers. gosastan, ultimately from Proto-Ir. (+ *ui-) *said- “to break, split;” cf.
Av. (+ *aua-) auua.siδ- “to split;” Baluci sist/sid-, sind- “to break, split;” Skt. ched “to split, to cut off;” Gk. sizo “to cut through, to split;” L. scindo “I tear up.”