noun, singular: archaeon
(2) One in the three-domain system (the other are Bacteria and Eukaryota) which includes halophiles (microorganisms that may inhabit extremely salty environments), methanogens (microorganisms that produce methane), and thermophiles (microorganisms that can thrive extremely hot environments)
Archaea or archaebacteria evolved separately from eubacteria and eukaryotes. They are similar with eubacteria in being prokaryotes and lacking distinct cell nucleus. They differ in terms of ribosomal structure, the possession of introns (in some species) and in membrane structure or composition. They are similar to eukaryotes in ways that archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes: notably the enzymes involved in transcription and translation.