An antonym is a word that has the opposite meaning of another word. For example, the antonym of hot is cold, and the antonym of happy is sad. Antonyms help us understand language better by providing contrast and clarity in communication. Antonyms can be categorized into different types, such as gradable antonyms, which allow for degrees of difference, like big and small, and complementary antonyms, which are absolute opposites, like alive and dead. Understanding antonyms enhances vocabulary and improves language skills.