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{{for|the journal|Taxon (journal)}}
{{Biological classification}}

A '''taxon''' (plural '''taxa'''), or '''taxonomic unit''', is a name designating an organism or group of [[organism]]s. In [[biological nomenclature]] according to [[Carl Linnaeus]], a taxon is assigned a [[taxonomic rank]] and can be placed at a particular level in a systematic hierarchy reflecting evolutionary relationships.

A distinction is to be made between taxa/taxonomy and classification/systematics. The former refers to biological names and the rules of naming. The latter refers to rank ordering of taxa according to presumptive evolutionary (phylogenetic) relationships.

Note: "Phylum" applies formally to any biological [[domain (biology)|domain]], but traditionally it was always used for animals, whereas "Division" was traditionally often used for [[plant]]s, [[fungus|fungi]], etc.

A simple  [[mnemonic|mnemonic phrase]] to remember the sequence of taxonomic levels is:
"Dignified Kings Play [[Chess]] On Fine Green Silk"; another, highly expedient example is "King Philip's Class Orders the Family Genius to Speak".

A prefix is used to indicate a ranking of lesser importance. The prefix ''super-'' indicates a rank above, the prefix ''sub-'' indicates a rank below. In zoology the prefix ''infra-'' indicates a rank below ''sub-''.  For instance:
:[[Superclass (biology)|Superclass]]
:[[Class (biology)|Class]]
:[[Subclass (biology)|Subclass]]
:[[Infraclass (biology)|Infraclass]]

Rank is relative, and restricted to a particular systematic schema. For example, [[liverworts]] have been grouped, in various systems of classification, as a family, order, class, or division (phylum). The use of a narrow set of ranks is challenged by users of [[cladistics]]; for example, the mere 10 ranks traditionally used between animal families (governed by the [[ICZN]]) and animal phyla (usually the highest relevant rank in taxonomic work) often cannot adequately represent the evolutionary history as more about a lineage's [[phylogeny]] becomes known. In addition, the class rank is quite often not an evolutionary but a [[phenetic]]al and [[paraphyletic]] group and as opposed to those ranks governed by the ICZN, can usually not be made monophyletic by exchanging the taxa contained therein. This has given rise to [[phylogenetic taxonomy]] and the ongoing development of the [[PhyloCode]], which is to govern the application of taxa to [[clade]]s.

==See also== 
*[[Alpha taxonomy]]
*[[Cladistics]]
*[[ICBN]]
*[[ICZN]]
*[[Rank (botany)]]
*[[Rank (zoology)]]
*[[Segregate (taxonomy)]]

[[Category:Scientific classification]]
[[Category:Zoological nomenclature]]
[[Category:Botanical nomenclature]]

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