What is the opposite of a synonym?

The opposite of a synonym is an antonym.

A synonym is a word with the same or a closely related meaning as another word, while an antonym is a word that expresses an opposite meaning. For example, hot and cold, early and late, and increase and decrease can form antonymic pairs.

Linguists commonly describe several kinds of antonyms. Gradable antonyms lie at opposite ends of a scale, as with hot and cold. Complementary antonyms form a binary contrast, such as odd and even. Relational antonyms describe the same relationship from opposing viewpoints, as in teacher and pupil or buy and sell.

Antonym does not always mean a perfect logical opposite. Context matters, and one word can have several antonyms depending on which sense is intended. For instance, light can contrast with dark when discussing brightness, or with heavy when discussing weight. A homonym instead refers to words that share spelling or pronunciation while having different meanings; a hyponym is a more specific term within a broader category. Neither is the general opposite of synonym.