A taxon (plural: taxa) is a group of one (or more) populations of organism(s), which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a taxonomic group is done by a taxonomist with the science of taxonomy. It is not uncommon for one taxonomist to disagree with another on what exactly belongs to a taxon, or on what exact criteria should be used for inclusion.
Taxonomists sometimes make a distinction between "good" (or natural) taxa and others that are "not good" (or artificial). Today it is common to define a good taxon as one that reflects evolutionary (phylogenetic) relationships, but this is not mandatory.
A taxon may be given a formal scientific name, the application of which is governed by one of the Nomenclature Codes, which set out rules to determine which scientific name is correct for that particular grouping.
Advocates of phylogenetic nomenclature, using cladistic methods, require taxa to be monophyletic, consisting of all descendants of some ancestor. They generally do not refer to taxa as their basic unit, but to "clades," a clade being a special form of taxon. However, even in traditional nomenclature, few taxonomists of our time would establish new taxa that they know to be paraphyletic. A famous example of a widely accepted taxon that is not also a clade is the "Reptilia."
Read more about Taxon: Definition, Ranks