The Collaborative International Dictionary of English v.0.48:

20 \20\ adj.
   1. one more than nineteen; denoting a quantity consisting of
      twenty items or units; -- representing the number twenty
      as Arabic numerals

   Syn: twenty, xx, score
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

20
    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]