A taxon (plural taxa) is an element of a taxonomy, e.g. in the scientific classification in biology. Taxa form a hierarchical scheme, each being broken down into subtaxa.
In traditional Linnaean taxonomy, taxa are ranked as follows:
The prefix super- indicates a rank above another, the prefix sub- indicates a rank below another, and the prefix infra- indicates a rank below sub-. For instance:
It has become generally accepted that taxa should contain an ancestral form and all or some of its descendants, though the validity of the latter category is increasingly debated.