WordNet (r) 3.0 (2006):

Archean aeon
    n 1: the time from 3,800 million years to 2,500 million years
         ago; earth's crust formed; unicellular organisms are
         earliest forms of life [syn: Archean, Archean eon,
         Archean aeon, Archeozoic, Archaeozoic, Archeozoic
         eon, Archaeozoic aeon]