Glossary entry

subspecies (sing. and pl.), n.
(1) The species-group rank below species; the lowest rank at which names are regulated by the Code.
(2) A taxon at the rank of subspecies.

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