Geologic Clock with events and periods

In general usage, an eon (sometimes spelled aeon) is a period of time arbitrarily designated by humans. Geologists refer to an eon as the largest subdivision of time on the geologic time scale. For example, the Phanerozoic Eon, which is about 550 million years long, covers the period of time during which animals with hard shells that fossilize would have been abundant.

An eon is composed of several geologic eras, which in turn are composed of geologic periods, which are composed of geologic epochs. We are currently in the Phanerozoic Eon, the Cenozoic Era, the Neogene Period, and the Holocene epoch. More recently, the Hadean, Archean, and Proterozoic "eras" have been considered eons. The geologic timescale in terms of eons, eras, and periods looks like this:

Ediacaran PaleoproterozoicMesoproterozoic