—adj. noting or pertaining to the earlier half of the Precambrian Era, from about 5 billion to 2.5 billion years ago, during which the earliest datable rocks were formed and from which the oldest known fossil forms, blue-green algae and bacteria, have been recovered. See table under geologic time.
—n. the Archeozoic division of geologic time or the rock systems formed then. Also,Archaeozoic.