Proterozoic

US: (prot′ər ə zō′ik, prō′tər-)

WordReference Random House Unabridged Dictionary of American English © 2025

Prot•er•o•zo•ic  (prot′ər ə zō′ik, prō′tər-), [Geol.]
adj. 

1. Geology noting or pertaining to the latter half of the Precambrian Era, from about 2.5 billion to 570 million years ago, characterized by the appearance of bacteria and marine algae;
   Algonkian.

n. 

2. Geology the Proterozoic division of geologic time or the rock systems formed then;
   Algonkian. See table under geologic time. 

• protero- + zo- + -ic 1905–10