Taxon

A taxon (plural taxa), or taxonomic unit, is a grouping of organisms (named or unnamed). Once named, a taxon will usually have a rank and can be placed at a particular level in a hierarchy.

A broad scheme of ranks in hierarchical order:

• Kingdom
• Phylum (animals or plants) or Division (plants)
• Class
• Order
• Family
• Genus
• Species
• Subspecies

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
• Class
• Subclass
• Infraclass

Do note that rank is relative, and restricted to the particular scheme used. For example, liverworts have been grouped, in various systems of classification, as a family, order, or a class. The use of ranks is challenged by users of cladistics and has led to the PhyloCode being proposed.