| Noun | 1. | geological period - a unit of geological time during which a system of rocks formed; "ganoid fishes swarmed during the earlier geological periods" geologic time, geological time - the time of the physical formation and development of the earth (especially prior to human history) geological era, era - a major division of geological time; an era is usually divided into two or more periods epoch - a unit of geological time that is a subdivision of a period and is itself divided into ages glacial epoch, glacial period, ice age - any period of time during which glaciers covered a large part of the earth's surface; "the most recent ice age was during the Pleistocene" |