An eon (or aeon) is a term in Earth science for the longest periods of time. It describes a part of the Earth's existence lasting hundreds of millions to billions of years.

A geologic eon is part of Earth's existence, made up of a number of eras of different lengths. A geologic era is made up of two or more shorter times called geologic periods. A geologic period is, in turn made up of yet shorter times called epochs.