or ae·on

[ ee-uhn, ee-on ]
/ ˈi ən, ˈi ɒn /
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an indefinitely long period of time; age.
the largest division of geologic time, comprising two or more eras.
Astronomy. one billion years.
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Origin of eon

see origin at aeon

Words nearby eon

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023


What does eon mean?

Eon is commonly used in a general way to refer to an indefinitely long period of time. Outside of science, people usually use eon as a way to exaggerate how long something took to happen or how long something lasted.

In the context of geology, eon refers to the largest division of geological time. Technically speaking, an eon is made up of two or more eras, which consist of several periods, which are divided into epochs.

All of these units of time vary as to their exact length, but most consist of at least millions of years, with eons lasting billions or hundreds of millions of years.

In the context of astronomy, an eon is one billion years.

Example: It took me eons to get a straight answer as to how long an eon actually is.

Where does eon come from?

The first records of the word eon referring to a very long span of time come from around 1650. It comes from the Greek word aiṓn, referring to an infinitely long period of time.

Geologists generally divide the entirety of Earth’s existence into three eons. The first and oldest is the Archean eon, lasting from about 3.8 to 2.5 billion years ago. It is thought that most of the planet’s continental masses formed during this time. The next is the Proterozoic eon, lasting from about 2.5 billion to 540 million years ago, when bacteria and marine algae are thought to have appeared. The last (and present) eon is the Phanerozoic eon, stretching from about 540 million years ago to the present. The Archaen and Proterozoic eons are often combined into the Precambrian eon.

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What are some other forms of eon?

  • aeon (British English spelling)

What are some synonyms for eon?

  • age (when eon is used in a general or figurative sense)

What are some words that often get used in discussing eon?


How is eon used in real life?

In everyday conversation, eon is usually used to exaggerate how long something took. When used in a scientific way, it’s most often used in geology and astronomy.



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True or False? 

When the word is used in a scientific context, an eon always consists of the exact same amount of time.

How to use eon in a sentence

British Dictionary definitions for eon

/ (ˈiːən, ˈiːɒn) /

the usual US spelling of aeon
geology the longest division of geological time, comprising two or more eras
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for eon

[ ēŏn′ ]

The longest division of geologic time, containing two or more eras.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.