The Collaborative International Dictionary of English v.0.48:

28 \28\ adj.
   1. denoting a quantity consisting of twenty-eight items or
      units; -- representing the number twenty-eight as Arabic
      numerals

   Syn: twenty-eight, xxviii
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

28
    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]