A geologic age is a time period on the geologic timescale delimited by major geologic or paleontologic events. A geologic age is a subdivision of a geologic era.

When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, there was no way to determine what time scale they represented. The age of the Earth and of the rock strata was the subject of scientific debate for over 100 years as various advances in other sciences continued to place the creation of the Earth further into the past. In the latter part of the 20th century, it became possible to assign relatively firm dates using radioactive dating. In the intervening century and a half, geologists and paleontologists devised two relative time scales. One scale -- termed the Geologic Timescale -- is used by both sciences. Geologists tend to talk in terms of Upper/Late, Lower/Middle and often Middle parts of periods or eras -- e.g. "Upper Jurassic", "Middle Cambrian". Paleontologists divide the same periods into sometimes regional faunal assemblages. For example, in North America the Late Neoproterozoic and Early Cambrian are grouped into a Waucoban series that is then subdivided into zones based on trilobites. The same Neoproterozoic timespan is split into Tommotian , Atdabanian and Botomian periods in East Asia and Siberia.

In 1977 The Global Commission on Stratigraphy started an effort to define global references (GSSPs) for geologic periods and faunal stages.