The Collaborative International Dictionary of English v.0.48:

18 \18\ adj.
   1. one more than seventeen; denoting a quantity consisting of
      one more than seventeen and one less than nineteen; --
      representing the number eighteen as Arabic numerals

   Syn: eighteen, xviii
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

18
    adj 1: being one more than seventeen [syn: eighteen, 18,
           xviii]
    n 1: the cardinal number that is the sum of seventeen and one
         [syn: eighteen, 18, XVIII]