noun A term (word or phrase) which is synonymous with others.
“Happy” is a synonym of “glad”.
noun Any of the various names that could in theory be used for a taxon in accordance with the relevant rules.
noun The subset of those names other than the one to be used according to the rules.
noun Any name that has been applied to a taxon other than the one to be used, including those not allowed by the rules such as misspellings or nomina nuda.
noun An alternative (often shorter) name defined for an object in a database.