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n. pl. ar·chae·a (-kē-ə)
Any of various prokaryotic microorganisms of the domain Archaea, being genetically distinct from bacteria and often living in habitats with extreme environmental conditions such as high temperature or salinity. Also called archaebacterium.
[New Latin Archaeon (back-formed singular of Archaea, domain name), from Greek arkhaion, neuter singular of arkhaios, ancient (in reference to the very ancient separation of the archaea and the eubacteria in evolutionary history); see archaic.]
Any of a group of microorganisms that resemble bacteria but are different from them in certain aspects of their chemical structure, such as the composition of their cell walls. Archaea usually live in extreme environments, such as very hot or salty ones. The archaea are considered a separate kingdom in some classification systems, but a division of the prokaryotes in others. Also called archaebacterium.