In biological classification, rank is the level (the relative position) in a taxonomic hierarchy. The most basic rank is that of species, the next most important is genus, and then family. Sometimes (but only rarely) the term "taxonomic category" is used instead of "rank".
The International Code of Zoological Nomenclature defines rank, in the nomenclatural sense, as:
The level, for nomenclatural purposes, of a taxon in a taxonomic hierarchy (e.g. all families are for nomenclatural purposes at the same rank, which lies between superfamily and subfamily). The ranks of the family group, the genus group, and the species group at which nominal taxa may be established are stated in Articles 10.3, 10.4, 35.1, 42.1 and 45.1. 
- ^ International Commission on Zoological Nomenclature (1999) International Code of Zoological Nomenclature. Fourth Edition. - International Trust for Zoological Nomenclature, XXIX + 306 pp.