eon

[ee-uhn, ee-on]
noun
1.
an indefinitely long period of time; age.
2.
the largest division of geologic time, comprising two or more eras.
3.
Astronomy. one billion years.
Also, aeon.


Origin:
see aeon

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
eon (ˈiːən, ˈiːɒn)
 
n
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
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline
Word Origin & History

eon
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
In nature, all experiments are rigorously tested over eons.
The water is contaminated naturally by sandstone, which has slowly leached
  radioactive contaminants over the eons.
The zone can shift over the eons as the star ages and becomes brighter and
  hotter.
As a result, the water would theoretically remain frozen for eons.
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