Precambrian
Concise Oxford English Dictionary © 2008 Oxford University Press:
Precambrian /priːˈkambrɪən/
▶adjective Geology relating to or denoting the earliest aeon of the earth's history, preceding the Cambrian period and ending about 570 million years ago (comprising the Archaean and Proterozoic aeons).
'Precambrian' also found in these Oxford entries:

