"word having the same sense as another," early 15c. (but usually in plural form before 18c., or, if singular, as synonyma), from Old French synonyme (12c.) and directly from Late Latin synonymum, from Greek synonymon "word having the same sense as another," noun use of neuter of synonymos "having the same name as, synonymous," from syn- "together, same" (see syn-) + onyma, Aeolic dialectal form of onoma "name" (from PIE root *no-men- "name").
"an antithetical word," 1867, coined to serve as opposite of synonym, from Greek anti "opposite, against" (see anti-) + onym "name" (from PIE root *no-men- "name"). Perhaps introduced to English in the book "Synonyms and Antonyms" (1867) by the Ven. C.J. Smith, M.A.
UNDER the head of Synonyms and Antonyms, Archdeacon Smith arranges words which form an antithesis to one another. The word "antonym" is, we believe, a new formation but useful. [Journal of Sacred Literature, July 1867]
French antonyme (1842), German antonym (by 1859) are older. The un-Greek alternative counterterm has been left to fade.
Proto-Indo-European root meaning "name."
It forms all or part of: acronym; allonym; ananym; anonymous; antonomasia; antonym; binomial; caconym; cognomen; denominate; eponym; eponymous; heteronym; homonym; homonymous; hyponymy; ignominious; ignominy; innominable; Jerome; matronymic; metonymy; metronymic; misnomer; moniker; name; nomenclature; nominal; nominate; noun; onomastic; onomatopoeia; paronomasia; paronym; patronym; patronymic; praenomen; pronoun; pseudonym; renown; synonym; synonymy; synonymous; toponym.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nama; Avestan nama; Greek onoma, onyma; Latin nomen; Old Church Slavonic ime, genitive imene; Russian imya; Old Irish ainm; Old Welsh anu "name;" Old English nama, noma, Old High German namo, Old Norse nafn, Gothic namo "name."