Glossary entry

An available name which either
(1) is objectively invalid (i.e. it is a junior homonym or a junior objective synonym of a potentially valid name, or must be rejected under the provisions of the Code, or has been suppressed by the Commission), or
(2) is subjectively invalid (because it is considered subjectively to be a junior synonym or to be inapplicable to a particular taxonomic taxon).

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