The Collaborative International Dictionary of English v.0.48:

xxix \xxix\, adj.
   The Roman number representing twenty-nine.

   Syn: twenty-nine, 29.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

xxix
    adj 1: being nine more than twenty [syn: twenty-nine, 29,
           xxix]
    n 1: the cardinal number that is the sum of twenty-eight and one
         [syn: twenty-nine, 29, XXIX]