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. Formerly, only one eon existed besides the Phanerozoic: the Precambrian. More recently, the Hadean, Archean, and Proterozoic "eras" of Precambrian time have been considered eons. The geologic timescale in terms of eons, eras, and periods looks like this:
The English word eon is derived from the Koine Greek word aion.
Although a proposal was made in 1957 to define an aeon to be a unit of time equal to one billion years (1 Ga), the idea was not approved as a unit of scientific measure and is seldom used for a specific period of time. Its more common usage is for any long, indefinite, period of time. The origin is from the Greek root "aion" for "age" or "life force." A similar Latin word "aevum" for age is still present in words such as Longevity and medieval. 
- International Stratigraphic Chart from the International Commission on Stratigraphy
- USA National Park Service
- Washington State University
- Web Geological Time Machine
- Math Words - An alphebetical index
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