The Collaborative International Dictionary of English v.0.48:

xxviii \xxviii\, adj.
   The Roman number representing twenty-eight.

   Syn: twenty-eight, 28.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

xxviii
    adj 1: being eight more than twenty [syn: twenty-eight, 28,
           xxviii]
    n 1: the cardinal number that is the sum of twenty-seven and one
         [syn: twenty-eight, 28, XXVIII]