In general usage, an eon (also spelled Aeon) is a very long period of time. Geologists refer to an eon as the largest period of geologic time, composed of several eras. For example, geologists refer to the Phanerozoic eon which is about 550 million years long and covers the period of time during which animals with hard shells that fossilize well have been abundant. Less frequently, they refer to a Precambrian eon that covers the four billion years of Earth history prior to the appearance of hardshelled animals.

Quantitatively, an eon is a period of time of 1,000,000,000 years.

See also

• The gnostic term Aeon
• The musician Ian B. Loveday has released recordings of techno music under the name, Eon.
• EON Productions, the makers of the James Bond films