noun, plural thirties.
a cardinal number, 10 times 3.
a symbol for this number, as 30 or XXX.
a set of this many persons or things.
Printing, Journalism. 30-dash.
thirties, the numbers, years, degrees, or the like, from 30 through 39, as in referring to numbered streets, indicating the years of a lifetime or of a century, or referring to degrees of temperature: He works in the East Thirties. She must be in her thirties. The temperature was in the thirties yesterday.
amounting to 30 in number.

before 900; Middle English thritty, Old English thrītig, equivalent to thrī three + -tig -ty1; cognate with Dutch dertig, German dreissig, Old Norse thrjātīu Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
thirty (ˈθɜːtɪ)
n , pl -ties
1.  See also number the cardinal number that is the product of ten and three
2.  a numeral, 30, XXX, etc, representing this number
3.  (plural) the numbers 30--39, esp the 30th to the 39th year of a person's life or of a century
4.  the amount or quantity that is three times as big as ten
5.  something representing, represented by, or consisting of 30 units
6.  a.  amounting to thirty: thirty trees
 b.  (as pronoun): thirty are broken
[Old English thrītig; see three, -ty1]

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

c.1413, metathesis of O.E. þritig, from þri, þreo "three" + -tig "group of ten" (see -ty (1)). Cf. O.Fris. thritich, O.S. thritig, Du. dertig, O.H.G. drizzug, Ger. dreissig. Thirty Years' War (1842) was a religious power struggle waged 1618-48, mainly on Ger.
soil. The symbol -30- as printer and telegrapher's code to indicate the last sheet or line of copy or a dispatch is recorded from 1895. In 20c. jargon of journalism, it came to be a traditional sign-off signal and slang word for "the end."
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences for thirty
Each of the thirty legislative districts has one senator and two
Thirty percent of the profits from gambling machines come from problem gamblers.
Speer surrounded the site with one hundred and thirty antiaircraft searchlights.
Thirty minutes after the hour is commonly called half past in both bre and ame.
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