In biological taxonomy, a domain ( or ) (Latin: regio), also superkingdom or empire, is the highest taxonomic rank of organisms in the three-domain system of taxonomy designed by Carl Woese et.al. in 1990.
According to this system, the tree of life consists of three domains: Archaea, Bacteria, and Eukarya. The first two are all prokaryotic microorganisms, or single-celled organisms whose cells have no nucleus. All life that has a nucleus and membrane-bound organelles, and multicellular organisms, is included in the Eukarya.
The term "domain" was proposed by Carl Woese, Otto Kandler and Mark Wheelis (1990) in a three-domain system. This term represents a synonym for the category of dominion (Lat. dominium), introduced by Moore in 1974. However, only Stefan Luketa uses the term "dominion". He created two additional domains ("dominions") for Prions and Viruses.
Each of these three domains contains unique rRNA. This forms the basis of the three-domain system. While the presence of a nuclear membrane differentiates the Eukarya from the Archaea and Bacteria, both of which lack a nuclear membrane, distinct biochemical and RNA markers differentiate the Archaea and Bacteria from each other.
Archaea are prokaryota cells, typically characterized by membrane lipids that are branched hydrocarbon chains attached to glycerol by ether linkages. The presence of these other linkages in Archaea adds to their ability to withstand extreme temperatures and highly acidic conditions, but many archeae live in mild environments. Halophiles, organisms that thrive in highly salty environments, and hyperthermophiles, organisms that thrive in extremely hot environments, are examples of Archaea.
Archaea evolved many cell sizes, but all are relatively small. Their size ranges from 0.1 ?m to 15 ?m diameter and up to 200 ?m long. They are about the size of bacteria, or similar in size to the mitochondria found in eukaryotic cells. Members of the genus Thermoplasma are the smallest of the Archaea.
Even though bacteria are prokaryotic cells just like Archaea, their membranes are made of oligonucleotide acid attached to glycerol by ester linkages. Cyanobacteria and mycoplasmas are two examples of bacteria. They characteristically do not have ether linkages like Archaea, and they are grouped into a different category--and hence a different domain. There is a great deal of diversity in this domain. Confounded by that diversity and horizontal gene transfer, it is next to impossible to determine how many species of bacteria exist on the planet, or to organize them in a tree-structure, without cross-connections between branches.
Members of the domain Eukarya--called eukaryotes--have membrane-bound organelles (including a nucleus containing genetic material) and are represented by five kingdoms: Plantae, Protista, Animalia, Chromista, and Fungi.
The three-domain system does not include any form of non-cellular life. As of 2011 there was talk about nucleocytoplasmic large DNA viruses possibly being a fourth domain of life, a view supported by researchers in 2012. Stefan Luketa proposed a five-domain system in 2012, adding Prionobiota (acellular and without nucleic acid) and Virobiota (acellular but with nucleic acid) to the traditional three domains.
Alternative classifications of life include: