Definition - A word that has the opposite meaning of another word in the same language.

Example -
The word down is an antonym of the word up.

Etymology -
The word derives from the Greek anti, opposite + onym, name. It was coined in 1870 specifically to serve as the antonym of the word synonym. It replaced the word counterterm.

Oxford English Dictionary -
Its first citation is from 1870:
"The Etymology of the word … [Gk: antonymia] merely expresses the idea of one word in substitution for, which in matters of verbal debate, is equivalent practically to opposition to another; a double force which, in addition to its analogy to Synonym, seemed to render Antonym a preferable word to Counterterm ."
(C. J. Smith Syn. & Antonyms Pref. 6 )

Please comment