Archaea (; from Greek αρχαία, "old ones"; singular Archaeum, Archaean, or Archaeon), also called Archaebacteria (), is a major division of living organisms. Although there is still uncertainty in the exact phylogeny of the groups, Archaea, Eukaryota and Bacteria are the fundamental classifications in what is called the three-domain system. Like bacteria, Archaea are single-celled organisms lacking nuclei and are therefore classified as Prokaryota — known as Monera in the five-kingdom taxonomy. They were originally described in extreme environments, but have since been found in all types of habitats.
A single organism from this domain has been called an "archaean." Furthermore, this biologic term is also used as an adjective.

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