Definition of Antonyms. An antonym is a word that means the opposite of another word. For instance, the antonym of 'hot' may be 'cold.' The root words for the word 'antonym' are the words 'anti,' meaning 'against' or 'opposite,' and 'onym,' meaning 'name.'. In order to better understand antonyms, let's take a look at what the word 'synonym' means. A synonym is a word that has a similar meaning to or exactly the same meaning as another word. Synonyms and antonyms are exactly the opposite. Understanding and using antonyms can improve our writing skills and vocabulary knowledge.