relating to or denoting the aeon that (in some schemes) constitutes the earliest part of the Precambrian, preceding the Archaean aeon. It extended from the origin of the earth to about 4,000 million years ago, and has left no identifiable rocks.
(as noun the Priscoan) the Priscoan aeon.
formed irregularly from Latin priscus 'ancient' + -an
Priscoan in other Oxford dictionaries
Definition of Priscoan in the US English dictionary