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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:

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.

Topics in phylogenetics
Relevant fields: phylogenetics | computational phylogenetics | molecular phylogeny | cladistics
Basic concepts: synapomorphy | phylogenetic tree | phylogenetic network | long branch attraction
Phylogeny inference methods: maximum parsimony | maximum likelihood | neighbour joining | UPGMA