Also: a taxonomic group (or unit, or entity) of any rank: a taxonomic group that is sufficiently distinct to be worthy of being distinguished by name and to be ranked in a definite category.

Also: a named phenon

taxonomic category: designates rank or level in a hierarchic classification. It is a class, the members of which are all taxa assigned to a given rank.

taxonomic species: a general expression for any taxon which has been called a species and given a specific name available under the International Code for Zoological Nomenclature.

The essential of a taxon is that it has a name so it's really quite simple. if formal.