D [1] (also d) NOUN (pl. Ds or D's) 1) the fourth letter of the alphabet. 2) denoting the fourth item in a set. 3) Music the second note of the diatonic scale of C major. 4) the Roman numeral for 500. [ORIGIN: understood as half of CI, an earlier form of M (= 1,000).]
D [2] ABBREVIATION 1) (in the US) Democrat or Democratic. 2) depth (in the sense of the dimension of an object from front to back). 3) (with a numeral) dimension(s) or dimensional. 4) (in tables of sports results) drawn.

English terms dictionary. 2015.