synonym (n.)

"word having the same sense as another," early 15c., synoneme, sinonyme, from Old French synonyme (12c.) and directly from Late Latin synonymum, from Greek synōnymon "word having the same sense as another," noun use of neuter of synōnymos "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").

Opposite of antonym. In Middle English also as the name of a 7c. book by Isidore, a dialogue between Man and Reason who describe differently the same situations. Until 18c. usually in plural form, synonymes, or, if singular, synonyma. The modern form of the word "scarcely makes its appearance, except in dictionaries, till the close of the 18th century" [OED, 2nd edition, print, 1989].