三域系统是由美国微生物学家和生物物理学家卡尔·乌斯等人在1977年提出的细胞生命形式的分类，将原核生物分成了两大类，起初称为真细菌和古细菌。乌斯依据16S 核糖体RNA串行上的差别，认为这两类生物和真核生物一起从一个具有原始遗传机制的祖先（Last universal ancestor）分别演化而来，因此将三者各置为一个「域」，作为比「界」高一级的分类系统，并分别命名为细菌域、古菌域和真核域。乌斯最初使用术语“界”来指代三个主要种系发生分组，并且这个命名法被广泛使用，一直到术语“域”在1990年被采用。

Phylogenetic nomenclature, often called cladistic nomenclature, is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a type, which can be a specimen or a taxon of lower rank, and a diagnosis, a statement intended to supply characters that differentiate the taxon from others with which it is likely to be confused. Phylogenetic nomenclature is currently not regulated, but the International Code of Phylogenetic Nomenclature (PhyloCode) is intended to regulate it once it is ratified.