1) Any of the unicellular microorganisms that is genetically distinct from bacteria and eukaryotes, and often inhabiting extreme environmental conditions.

(2) One in the three-domain system (the other are Bacteria and Eukarya) which includes halophiles (microorganisms that may inhabit extremely salty environments), methanogens (microorganisms that produce methane), and thermophiles (microorganisms that can thrive extremely hot environments)

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