1Relating 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.

1. 1.1as noun the Priscoan The Priscoan aeon.
   前寒武纪初期地质宙