Eon
In general usage, an eon 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.Quantitativly, eon refers to a period of time of 1,000,000,000 years.