synonym vs. antonym

a word having the same or nearly the same meaning as another word in the same language, as happy, joyful, elated. A dictionary of synonyms and antonyms (or opposites), such as Thesaurus.com, is called a thesaurus.

a word or expression accepted as another name for something, as Arcadia for pastoral simplicity or Wall Street for U.S. financial markets; metonym.

Biology. one of two or more scientific names applied to a single taxon.

a word opposite in meaning to another. Fast is an antonym of slow. Compare synonym ( def 1 ).