The Collaborative International Dictionary of English v.0.48:

xxiii \xxiii\, adj.
   The Roman number representing twenty-three.

   Syn: twenty-three, 23.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

xxiii
    adj 1: being three more than twenty [syn: twenty-three, 23,
           xxiii]
    n 1: the cardinal number that is the sum of twenty-two and one
         [syn: twenty-three, 23, XXIII]