A third kingdom evolutionarily distinct from bacteria and eukaryotes, but closest to eukaryotes according to RNA analysis; tend to live in extreme environments; include sulfur, heat, salt, acid, and pressure loving "bacteria".

One of the three domains of life (the others being BACTERIA and Eukarya), formerly called Archaebacteria under the taxon Bacteria, but now considered separate and distinct. They are characterized by: (1) the presence of characteristic tRNAs and ribosomal RNAs; (2) the absence of peptidoglycan cell walls; (3) the presence of ether-linked lipids built from branched-chain subunits; and (4) their occurrence in unusual habitats. While archaea resemble bacteria in morphology and genomic organization, they resemble eukarya in their method of genomic replication. The domain contains at least four kingdoms: CRENARCHAEOTA; EURYARCHAEOTA; NANOARCHAEOTA; and KORARCHAEOTA.

Single-celled organisms lacking nuclei, similar to prokaryotes in most aspects of cell structure and metabolism. However, their genetic transcription and translation do not show the typical bacterial features, but are extremely similar to those of eukaryotes.