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.
      [1913 Webster]

   2. A symbol representing twenty units, as 20, or xx.
      [1913 Webster]

WordNet (r) 3.0 (2006):

twenties
    n 1: the time of life between 20 and 30 [syn: twenties, mid-
         twenties]
    2: the decade from 1920 to 1929 [syn: twenties, 1920s]