A word that is opposite in meaning to another word is called an antonym.
The term comes from Greek: anti means “against” or “opposite,” while onyma means “name.” Antonyms create contrasts such as hot and cold, early and late, or accept and reject. They are a type of semantic relationship, meaning the connection concerns word meaning rather than spelling or sound.
Not all antonyms work in exactly the same way. Gradable antonyms describe opposite ends of a scale, as with hot and cold. Complementary antonyms present mutually exclusive choices, such as alive and dead. Relational antonyms describe opposing roles or directions, such as teacher and student or buy and sell.
A common mix-up is confusing antonyms with synonyms. Synonyms have similar meanings, such as quick and fast. Homonyms instead share spelling or pronunciation while having different meanings, and a pseudonym is an assumed name. A word can also have more than one antonym depending on its sense.