The Collaborative International Dictionary of English v.0.48:

xxiv \xxiv\, adj.
   The Roman number representing twenty-four.

   Syn: twenty-four, 24.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

xxiv
    adj 1: being four more than twenty [syn: twenty-four, 24,
           xxiv]
    n 1: the cardinal number that is the sum of twenty-three and one
         [syn: twenty-four, 24, XXIV, two dozen]