(strictly) A word whose meaning is the same as that of another word.
A word or phrase with a meaning that is the same as, or very similar to, another word or phrase.
Any of the formal names for a taxon, including the valid name (i.e. the senior synonym).
Any name for a taxon, usually a validly published, formally accepted one, but often also an unpublished name.
An alternative (often shorter) name defined for an object in a database.