The Collaborative International Dictionary of English v.0.48:

13 \13\ adj.
   1. denoting a quantity consisting of one more than twelve and
      one less than fourteen; -- representing the number
      thirteen as Arabic numerals

   Syn: thirteen, xiii
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

13
    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]