Source : Webster's Revised Unabridged Dictionary (1913)

Antonym \An"to*nym\, n. [Gr. ? a word used in substitution for
   another; ? + ?, ?, a word.]
   A word of opposite meaning; a counterterm; -- used as a
   correlative of synonym. [R.] --C. J. Smith.

Source : WordNet®

antonym
     n : two words that express opposing concepts; "to him the
         opposite of gay was depressed" [syn: {opposite word}, {opposite}]