a cardinal number, ten plus eight.
a symbol for this number, as 18 or XVIII.
a set of this many persons or things.
amounting to 18 in number.

before 1000; Middle English ehtetene, Old English eahtatēne; cognate with Old Norse āttjān, German achtzehn. See eight, -teen

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eighteen (ˈeɪˈtiːn)
1.  See also number the cardinal number that is the sum of ten and eight and the product of two and nine
2.  a numeral, 18, XVIII, etc, representing this number
3.  the amount or quantity that is eight more than ten
4.  something represented by, representing, or consisting of 18 units
5.  (functioning as singular or plural) a team of 18 players in Australian Rules football
6.  a.  amounting to eighteen: eighteen weeks
 b.  (as pronoun): eighteen of them knew
[Old English eahtatēne; related to Old Norse attjan, Old High German ahtozehan]

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

O.E. eahtatene, eahtatyne; see eight + -teen.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Eighteen years later, not-labeling has caused no end of problems for the
  biotech industry.
For eighteen months this pretty little nurse was beside herself with worry.
No more than a foot wide and eighteen inches long, the stand had a sacred air,
  as if it belonged on an altar.
Eighteen months ago, he points out, he had to explain to people what e-books
  and e-readers were.
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