a word or expression accepted as another name for something, as Arcadia for pastoral simplicity; metonym.
3.
Biology. one of two or more scientific names applied to a single taxon.
Origin: 1400–50; < Latin synōnymum < Greek synṓnymon, noun use of neuter of synṓnymossynonymous; replacing Middle English sinonyme < Middle French < Latin, as above
early 15c. (but rare before 18c.), from L. synonymum, from Gk. synonymon "word having the same sense as another," noun use of neut. of synonymos "having the same name as, synonymous," from syn- "together, same" + onyma, Aeolic dialectal form of onoma "name" (see name).