- For the journal, see International Association for Plant Taxonomy.
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:
- Domain
- Kingdom
- Phylum (animals or plants) or Division (plants)
- Class
- Order
- Family
- Genus
- Species
- Subspecies
A simple mnemonic phrase to remember the order is "King Philip Came Over From Great Spain" or "King Phill Classed Ordinary Families as Generous and Special".
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.