1 In the widely-used five-kingdom system for classifying organisms, a phylum within the subkingdom Archaea in the kingdom Bacteria.
2 In the three-domain classification, the more derived (see apomorph) of the two kingdoms (sometimes called subdomains) within the domain Archaea. The Euryarchaeota contains a broad range of phenotypes, including methanogens, halophiles, hyperthermophiles, acidophiles, and sulphur-reducing organisms. Genetically, members of the Euryarchaeota are more different from members of the domains Eukarya and Eubacteria than are members of the Crenarchaeota.