分类单元（分类群，英语：Taxon）是指分类学上的一个群体，不管处哪一个分类阶层（taxonomic rank），称此群体为分类群。

各个分类阶层皆可能具多个分类群，而其整体亦为一个分类群。分类群可以给定一个名字，但若指整个分类阶层，则其命名便要受国际命名法规规范，例如种应有种的学名、纲需有纲的学名。

以槐为例，以下列出十个分类阶层，主要的分阶层（principal ranks） 指界、门、纲、目、科、属、种等七个，若有需要可以在七个主要的阶层之上或之下，再加合适的次级阶层（secondary ranks，例如科与属之间的族、组、系，种下的亚，或是亚科、亚属等等）。每个阶层皆有其特定的学名，学名则受到命名法规的规范。

In biology, a taxon (plural taxa; back-formation from taxonomy) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and given a particular ranking, especially if and when it is accepted or becomes established. It is not uncommon, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion. If a taxon is given a formal scientific name, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.