The Collaborative International Dictionary of English v.0.48:

xxi \xxi\, adj.
   The Roman number representing twenty-one.

   Syn: twenty-one, 21.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

xxi
    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]