[ee-uhn, ee-on]
an indefinitely long period of time; age.
the largest division of geologic time, comprising two or more eras.
Astronomy. one billion years.
Also, aeon.

see aeon

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
aeon or esp (US) eon (ˈiːən, ˈiːɒn, ˈiːən, ˈiːɒn)
1.  an immeasurably long period of time; age
2.  a period of one thousand million years
3.  (often capital) gnosticism one of the powers emanating from the supreme being and culminating in the demiurge
[C17: from Greek aiōn an infinitely long time]
eon or esp (US) eon
[C17: from Greek aiōn an infinitely long time]

eon (ˈiːən, ˈiːɒn)
1.  the usual US spelling of aeon
2.  geology the longest division of geological time, comprising two or more eras

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

1640s, from L. aeon, from Gk. aion "age, vital force, lifetime," from PIE base *aiw- "vital force, life, long life, eternity" (cf. Skt. ayu "life," Avestan ayu "age," L. aevum "space of time, eternity," Goth. aiws "age, eternity," O.N. ævi "lifetime," Ger. ewig "everlasting," O.E. a "ever, always").
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
eon   (ē'ŏn')  Pronunciation Key 
The longest division of geologic time, containing two or more eras.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Psychiatry is a eon behind any such revelation of the untoward ontology of nightmares.
We long to hold on to those rituals and traditions of eon that still resonate, while making new meaning from the ether.
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