Webster's Revised Unabridged Dictionary · 1913

D

D (dē)

1.
The fourth letter of the English alphabet, and a vocal consonant. The English letter is from Latin, which is from Greek, which took it from Phonician, the probable ultimate origin being Egyptian. It is related most nearly to t and th; as, Eng. deep, G. tief; Eng. daughter, G. tochter, Greek qyga`thr, Sanskrit duhitr. See Guide to Pronunciation, r178, 179, 229.
2.
(Music) The nominal of the second tone in the model major scale (that in C), or of the fourth tone in the relative minor scale of C (that in A minor), or of the key tone in the relative minor of F.
3.
As a numeral D stands for 500. in this use it is not the initial of any word, or even strictly a letter, but one half of the sign {not transcribed} (or {not transcribed} ) the original Tuscan numeral for 1000.