The Collaborative International Dictionary of English v.0.48:

Twenty \Twen"ty\ (tw[e^]n"t[y^]), a. [OE. twenty, AS.
   tw[=e]ntig, twentig; akin to OFries. twintich, OS.
   tw[=e]ntig, D. & LG. twintig, OHG. zweinzug, G. zwanzig,
   Goth. twai tigjis; that is, two tens. See Twain, Two, and
   Ten.]
   1. One more than nineteen; twice ten; as, twenty men.
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   2. An indefinite number more or less that twenty. --Shak.
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            Maximilian, upon twenty respects, could not have
            been the man.                         --Bacon.
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The Collaborative International Dictionary of English v.0.48:

Twenty \Twen"ty\, n.; pl. Twenties.
   1. The number next following nineteen; the sum of twelve and
      eight, or twice ten; twenty units or objects; a score.
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   2. A symbol representing twenty units, as 20, or xx.
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WordNet (r) 3.0 (2006):

twenty
    adj 1: denoting a quantity consisting of 20 items or units [syn:
           twenty, 20, xx]
    n 1: the cardinal number that is the sum of nineteen and one
         [syn: twenty, 20, XX]
    2: a United States bill worth 20 dollars [syn: twenty dollar
       bill, twenty]