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D (iou)



D, d. [di:]
The fourth letter of the modern English alphabet and of the ancient Roman one, corresp. to Greek delta, Hebrew daleth. The sound normally represented by the letter is a voiced alveolar plosive consonant. Pl. D's, Ds. See also DEE noun.
I.
The letter and its sound.
The shape of the letter.
D-lock a D-shaped mechanism used to secure a parked bicycle or motorcycle, consisting of a U-shaped bar and crosspiece of solid metal. D-ring a D-shaped metal ring through which a strap etc. can pass. D-shaped adjective having a shape or a cross-section like the capital letter D.
b. Billiards etc. The semicircle marked on the table in the baulk area, with its diameter part of the baulk line, from which a player must play after retrieving the cue-ball by hand.
II. Symbolical uses.
Used to denote serial order; applied e.g. to the fourth group or section, sheet of a book, etc.
Music. (Cap. D.) The second note of the diatonic scale of C major. Also, the scale of a composition with D as its keynote.
The fourth hypothetical person or example.
Math. (Usu. italic d.) In calculus, used before variables to indicate a derivative, e.g. in dy/dx, the derivative of y with respect to x.
(Usu. cap. D.) Designating the fourth-highest class (of academic marks, population as regards affluence, etc.).
The roman numeral for 500.
Chemistry. Orig. italic d, now only as a small capital D: applied to (a compound having) a configuration about an asymmetric carbon atom analogous to that of an arbitrarily chosen compound (now D-glyceraldehyde for organic compounds). Also, as D(+), D(), further denoting respectively dextro- or laevorotation of polarized light by the compound. [Extended use of d = dextrorotatory.]
D-layer, the lowest stratum of the ionosphere.
III.
Abbrevs.: D. = (US) Democrat; Distinguished (in decorations); Doctor (in academic degrees). D = defence (in D-notice, an official request to British news editors not to publish items on specified subjects, for reasons of security); (Chemistry) deuterium; dimension, dimensional, (esp. in 3-D, three-dimensions, -dimensional). d. = (colloq.) damn (adjective & adverb), damned; daughter; day; (colloq.) decent (esp. in jolly d.); delete; departs; depth; deputy; died; (in former British currency) [Latin] denarius, -rii penny, pence. d = (as prefix) deci-; (Chemistry) (as prefix) dextrorotatory (cf. sense 9 above); (Physics & Chemistry) diffuse: orig. designating one of the four main series (S, P, D, F) of lines in atomic spectra, now more frequently applied to electronic orbitals, states, etc., possessing two units of angular momentum (as d-electron, d-orbital, etc.); (Particle Physics) a quark flavour associated with a charge of 1 / 3 (cf. DOWN adjective 5).