Glossary entry

(1) A name which, under the provisions of the Code, cannot be used as a valid name and which is set aside in favour of another name.
(2) A name which, as a matter of taxonomic judgment, is either treated as a junior subjective synonym (q.v.) of a name used as valid or is believed not to be applicable to the taxon under consideration.

Source:
International Commission on Zoological Nomenclature (1999) International Code of Zoological Nomenclature. Fourth Edition. - International Trust for Zoological Nomenclature, XXIX + 306 pp.