Definition of "synonym"

(semantics, strictly) A word whose meaning is the same as that of another word.  examples

(semantics, loosely) A word or phrase with a meaning that is the same as, or very similar to, another word or phrase.  quotations  examples

(zoology) Any of the formal names for a taxon, including the valid name (i.e. the senior synonym).

(taxonomy, botany) Any name for a taxon, usually a validly published, formally accepted one, but often also an unpublished name.  examples

(databases) An alternative (often shorter) name defined for an object in a database.  quotations  examples