What's the term for a word that is the opposite of another word?

An antonym is a word that means the opposite of another word. For example, hot and cold are antonyms, as are good and bad. Antonyms can be all types of words: verbs, nouns, adjectives, adverbs, and even prepositions. The term antonym comes from antonymy, which is the technical grammar term for words that have contradictory meanings—but you can think of antonyms as opposites. Antonyms are closely related to synonyms, which are words that have identical meanings. While antonyms have opposite meanings, synonyms have the same meaning.

Source: Grammarly