| Definition -
A word that has the opposite meaning of another word.
1. It is what antonyms were called before the word antonym was coined in about 1870.
Example - The counterterm of the word up is down.
Oxford English Dictionary - The term's first citation is from 1870:
"The Etymology of the word … a 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)