In biology, a taxon is a group of one or more populations of an organism or organisms seen by taxonomist to form a unit. Although neither is required, a taxon is usually known by a particular name and given a particular ranking, especially if and when it is accepted or becomes established. It is not uncommon, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion. If a taxon is given a formal scientific name, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.

An example of a well-established taxon that is not also a clade is the class Reptillia , the reptiles; birds are descendants of reptiles but are not included in the Reptilia. The term taxon was first used in 1926 by Adolf Meyer for animal groups

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