noun, plural eighties.
a cardinal number, ten times eight.
a symbol for this number, as 80 or LXXX.
a set of this many persons or things.
eighties, the numbers, years, degrees, or the like, from 80 through 89, as in referring to numbered streets, indicating the years of a lifetime or of a century, or degrees of temperature.
amounting to 80 in number.

before 850; Middle English eighteti, Old English eahtatig. See eight, -ty1

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eighty (ˈeɪtɪ)
n , pl -ies
1.  See also number the cardinal number that is the product of ten and eight
2.  a numeral, 80, LXXX, etc, representing this number
3.  (plural) the numbers 80--89, esp a person's age or the year of a particular century
4.  the amount or quantity that is eight times as big as ten
5.  something represented by, representing, or consisting of 80 units
6.  a.  amounting to eighty: eighty pages of nonsense
 b.  (as pronoun): eighty are expected
[Old English eahtatig; related to Old Frisian achtig, Old High German ahtozug]

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. hundeahtatig, with from hund- "ten." See eight + -ty (1). Modern form attested by late 13c. Eighties in reference to a decade of years is recorded from 1883.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Eighty percent of the lava was produced during a single pulse of eruptions, she
Eighty-seven percent of the peninsula's glaciers have been retreating during
  the same period, he added.
Eighty-six adult pandas have also been confirmed safe.
See tractors and other farm equipment spanning the last eighty years.
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