geologic time, geological time - the time of the physical formation and development of the earth (especially prior to human history)

Age of Mammals, Cenozoic, Cenozoic era - approximately the last 63 million years

Age of Reptiles, Mesozoic, Mesozoic era - from 230 million to 63 million years ago

Paleozoic, Paleozoic era - from 544 million to about 230 million years ago

eon, aeon - the longest division of geological time

geological period, period - a unit of geological time during which a system of rocks formed; "ganoid fishes swarmed during the earlier geological periods"