Prot·er·o·zo·ic (prŏt′ər-ə-zōĭk, prō′tər-)
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.
The Proterozoic Eon.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.