The Collaborative International Dictionary of English v.0.48:

xxvii \xxvii\, adj.
   The Roman number representing twenty-seven.

   Syn: twenty-seven, 27.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

xxvii
    adj 1: being seven more than twenty [syn: twenty-seven, 27,
           xxvii]
    n 1: the cardinal number that is the sum of twenty-six and one
         [syn: twenty-seven, 27, XXVII]