taxon

A taxon is a unit used in biological classification to group organisms or other taxa that are regarded as forming a coherent unit for study and reference. A taxon can be any named group at any rank in the taxonomic hierarchy, from species and genus up to family, order, class, phylum, kingdom, or domain. The term also applies to clades defined by evolutionary relationships, which may not align with traditional ranks.

Taxa are defined through circumscription, which determines exactly which organisms are included, and delimitation, which clarifies the boundaries of the group. The status and composition of a taxon can change with new data, methods, or interpretations. Names and their authorship are governed by formal codes of nomenclature and require careful publication and, for many taxa, a designated type. In species-level work, for example, a holotype or other type specimen anchors the name to a concrete example.

Nomenclature codes provide the rules for naming and ranking. The International Code of Zoological Nomenclature (ICZN) governs animals; the International Code of Nomenclature for algae, fungi, and plants (ICN) governs plants, algae, and fungi; and the International Code of Nomenclature of Prokaryotes (ICNP) governs bacteria and archaea. These codes cover issues such as valid publication, priority, and the designation of type material, aiming to ensure stable and unique names.

In contemporary taxonomy, there is emphasis on monophyletic groupings, where taxa reflect common ancestry. Taxonomic concepts and circumscription continue to evolve with new data from morphology, genetics, and computational analyses.