Latin taxon, plural taxa; from Ancient Greek τάξις, "order, arrangement, organization".

A taxon is a qualifying unit in the systematics of organisms. It unites fishes by degree of relatedness. The main taxa are subspecies, species, genus, family, order, class, type.

Taxonomy is the section of systematics concerned with the study of the subordination of taxonomic categories.

In modern biological classifications, taxa form a hierarchical system: each taxon, on the one hand, consists of one or more taxa of a lower level of generality, while at the same time each taxon is part of another taxon - a group of a higher level of generality. Such a hierarchical system is called a taxonomic hierarchy, and its different levels are called taxonomic ranks.