any member of the Archaea, a domain of prokaryotic microorganisms, distinguished from bacteria on molecular phylogenetic grounds and often found in hostile environments, such as volcanic vents and hot springs
Archaean in British English
or especially US Archean (ɑːˈkiːən)
adjective
1.
of or relating to the highly metamorphosed rocks formed in the early Precambrian era
noun
2.
the earlier of two divisions of the Precambrian era, during which the earliest formsof life are assumed to have appeared