Prot·er·o·zo·ic 
(prŏt′ər-ə-zōĭk, prō′tər-)
(prŏt′ər-ə-zōĭk, prō′tər-)adj.
Of, relating to, or being the most recent of the two divisions of Precambrian time, from about 2.5 billion to 542 million years ago, marked by the formation of stable continents, the buildup of oxygen, and the appearance of the first multicellular eukaryotic life forms. See Table at geologic time.
n.
The Proterozoic Eon.
[Greek proteros, earlier, former; see per1 in the Appendix of Indo-European roots + -ZOIC.]