The Collaborative International Dictionary of English v.0.48:

21 \21\ adj.
   1. one more than twenty; twenty plus one more; denoting a
      quantity consisting of twenty-one items or units; --
      representing the number twenty-one as Arabic numerals

   Syn: twenty-one, xxi
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

21
    adj 1: being one more than twenty [syn: twenty-one, 21,
           xxi]
    n 1: the cardinal number that is the sum of twenty and one [syn:
         twenty-one, 21, XXI]