Proterozoic eon pro·te·ro·zoiceon uncommon noun 1. from 2,500 to 544 million years ago; bacteria and fungi; primitive multicellular organisms SynonymsProterozoicProterozoic eonProterozoic aeon Related words Broader terms eon Parts Precambrian WordNet 2.1 (Princeton University). WordNet 2.1 Copyright Princeton University. All rights reserved.
