n. A word having the same signification as another; one of two or more words which have the same meaning; by extension, a word having nearly the same meaning as another; one of two or more words which in use cover to a considerable extent the same ground: the opposite of antonym.n. A word of one language which corresponds in meaning with a word in another language. See heteronym, 2, paronym, 2, and the quotation from Camden under synonymize.n. In natural history, a systematic name having the same, or approximately the same, meaning or application as another which has superseded it; a technical name which, by the rules of nomenclature, is not tenable.