Archean

(noun) the time from 3,800 million years to 2,500 million years ago; earth's crust formed; unicellular organisms are earliest forms of life. Synonyms: Archean eon, Archean aeon, Archeozoic, Archaeozoic, Archeozoic eon, Archaeozoic aeon.

(adj) of or relating to the earliest known rocks formed during the Precambrian Eon. Synonyms: archean, archaean.