In biology, domain (also superregnum, superkingdom, or empire) is the top-level taxon of organisms in scientific classification, higher than a kingdom. Domain (or its synonyms) is the most inclusive of these biological groupings. The names are arbitrary, but the arrangement reflects the fundamental evolutionary differences in the genomes. There are several modern alternative domain classifications of life. Among them are:
- The two-empire system, with top-level groupings of Prokaryota (or Monera) and Eukaryota empires.
- The six-kingdom system with top-level groupings of Protista, Archaebacteria, Eubacteria, Fungi, Plantae, and Animalia.
and the most recent,
- The three-domain system of Carl Woese, introduced in 1990, with top-level groupings of Archaea, Bacteria, and Eukaryota domains.
As these groupings depend primarily on the analysis of genetic sequence data, additional proposed arrangements are to be expected.