The Collaborative International Dictionary of English v.0.48:

xiii \xiii\, adj.
   1. the Roman number symbolizing the value thirteen. Used
      after a noun it may symbolize the ordinal number; as,
      Superbowl XIII.

   Syn: thirteen, 13.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

xiii
    adj 1: being one more than twelve [syn: thirteen, 13,
           xiii]
    n 1: the cardinal number that is the sum of twelve and one [syn:
         thirteen, 13, XIII, baker's dozen, long dozen]