The term for a word that means the opposite of another word is an antonym.
The word comes from Greek roots: anti- means “against” or “opposite,” while onyma means “name.” Examples include hot and cold, early and late, and accept and reject. An antonym is defined by a relationship between meanings, so a word can have different antonyms depending on its sense.
Linguists commonly distinguish several kinds of antonyms. Gradable antonyms lie at opposite ends of a scale, such as small and large. Complementary antonyms present an either-or contrast, such as alive and dead. Relational antonyms describe opposing roles or perspectives, such as buy and sell or teacher and student.
Antonym is often confused with synonym, which means a word with a similar meaning. A homonym instead shares pronunciation or spelling with another word while having a different meaning, and an acronym is formed from initial letters or parts of words. These terms describe different language relationships.