An antonym is a word that means the opposite of another word. There are three types of antonyms: graded antonyms which express relationships along a continuum (e.g. good-bad), complementary antonyms which represent the only two opposite possibilities with no middle ground (e.g. true-false), and relational antonyms which require both opposites to exist (e.g. husband-wife). The document provides examples of antonym word pairs and exercises to identify antonyms.