Taxon

A taxon is a group of organisms recognized as a unit in taxonomy. A taxonomic rank specifies a position in the classification hierarchy; a taxon is the particular group placed there. Thus Felidae is a taxon at the rank of taxonomic family, while Carnivora is a taxon at the rank of taxonomic order.^[ref]

One organism can belong to many nested taxa. Membership in a species, its genus and its taxonomic family describes different levels of the same classification. A rank alone does not identify the group: the word family could refer to many distinct taxa.

For a constructed information-system example, let a family contain genera A and B, with three and five recognized species respectively. A species record can store its species identifier and follow parent relationships to recover its genus and family. If a later classification moves one species from A to B, the family's species total remains eight, while the genus totals change from 3 and 5 to 2 and 6. Group membership and the name of the rank are separate pieces of information.

A clade is specifically a group containing an ancestor and all its descendants. Calling a group a taxon does not by itself establish that it is a clade; that claim depends on its circumscription and the inferred phylogenetic tree.^[ref]