Taxonomic rank

In biological taxonomy, a taxonomic rank denotes the level that a group of organisms—either taxon or clade—occupies in a hierarchical system of classification, which is usually but not necessarily based on evolutionary relationships between those organisms. Some authors prefer to use the term nomenclatural rank, contending that, according to some definitions, the hierarchical rank occupied by a group of organisms is more accurately described as concept of nomenclature rather than taxonomy. In general, the most inclusive taxa (or clades), such as the Eukarya and Animalia, are assigned the highest ranks in a particular system of taxonomy, whereas the least inclusive ones, such as Homo sapiens, Bufo bufo, Tyrannosaurus rex, and Vulpes vulpes, are given the lowest ranks.

Ranks can be either "absolute", in which a pre-defined set of several descriptive terms (such as species, genus, tribe, family, order, class, phylum, kingdom, and domain) constitutes the set of allowable ranks for taxa; or "relative", where ranks are designated instead by an indented taxonomy in which the level of indentation reflects the rank. In a relative system, the rank of a taxon is not necessarily an inherent attribute of the taxon so much as an attribute of a particular indented/nested hierarchy defining the relations between different groups. Additionally, relative systems of taxonomical rank often allow indefinitely adding further nested taxa to allow finer distinctions to be captured as needed.

Rank-based codes (such as the Zoological Code, the Botanical Code, the Code for Cultivated Plants, the Prokaryotic Code, and the Code for Viruses) all require absolute ranks, but absolute ranks are not required in all nomenclatural systems—the PhyloCode (of phylogenetic nomenclature), for instance, does not require absolute ranks.