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Definition of Archaea
Medically reviewed by Dr. Otari Nergadze, Neurosurgeon | Updated: January 2026
Archaea: A unique group of microorganisms that are called bacteria (Archaeobacteria) but are genetically and metabolically different from all other known bacteria. They appear to be living fossils, the survivors of an ancient group of organisms that bridged the gap in evolution between bacteria and multicellular organisms (eukaryotes).