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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.[1]

Geologists have given names to all of the eons, eras, periods and epochs that they have found by looking at rocks. The names given to each eon describe something about that time, or the rocks that were made at that time.

For example, we are now in the Phanerozoic eon, which comes from the Greek words for life that we can see, that is, there were animals and plants big enough to see without a lens. This era started about 545 million years ago.

The four eons

Eon Beginning End[2] Name meaning[3]
Hadean 4600 million years ago 3900 million years ago Greek for "beneath the Earth"
Archaean 3900 million years ago 2500 million years ago Greek for "ancient"
Proterozoic 2500 million years ago 540 million years ago Greek for "earlier life"
Phanerozoic 540 million years ago Greek for "life we can see"