A word having a meaning opposite to that of another word. Antonym is the antonym of synonym.

Adjective: antonymous.

Antonymy is the sense relation that exists between words which are opposite in meaning. Antonymy is found most commonly (though not exclusively) among adjectives.

Linguists identify three types of antonymy: (1) Gradable antonyms, which operate on a continuum: (very) big, (very) small. Such pairs often occur in binomial phrases with and: (blow) hot and cold, (search) high and low. (2) Complementary antonyms, which express an either/or relationship: dead or alive, male or female. (3) Converse or relational antonyms, expressing reciprocity: borrow or lend, buy or sell, wife or husband."