Taxonomy

Taxonomy is the practice and science of classification. Taxonomy uses taxonomic units, known as taxa (singular taxon).

Applications
Originally the term taxonomy referred only to the classifying of organisms (now sometimes known as alpha taxonomy) or a particular classification of organisms. However, it has become fashionable in certain circles to apply the term in a wider, more general sense, where it may refer to a classification of things or concepts, as well as to the principles underlying such a classification.

Various biological taxonomies
Biological classification (sometimes known as "Linnaean taxonomy") is still generally the best known form of taxonomy. It uses taxonomic ranks, including, among others, (in order) Kingdom, Phylum, Class, Order, Family, Genus, Species (various mnemonic devices have been used to help people remember the list of "Linnaean" taxonomic ranks. See Zoology mnemonic). In zoology, the nomenclature for the more important ranks (superfamily to subspecies), including the allowed number of ranks, is strictly regulated by the ICZN Code, whereas there is more latitude for names at higher ranks. Taxonomy itself is never regulated, but is always the result of research in the scientific community. How researchers arrive at their taxa varies; depending on the available data, and resources, methods vary from simple quantitative or qualitative comparisons of striking features to elaborate computer analyses of large amounts of DNA sequence data.

Phylogenetics
Today, the alternative to the traditional rank-based biological classification is phylogenetic systematics, which is postulating phylogenetic trees (trees of descent), rather than focusing on what taxa to delimit. The best-known form of this is cladistics.

The results of cladistic analyses are often represented as cladograms. It is held by cladists that taxa (if recognized) must always correspond to clades, united by apomorphies (derived traits) which are discovered by a cladistic analysis. Some cladists hold that clades are poorly expressed in rank-based hierarchies and support the PhyloCode, a proposed ruleswork for the formal naming of clades, based on the model of the ICZN, ICBN etc. in rank-based nomenclature.