The Collaborative International Dictionary of English v.0.48:

Eighteen \Eight"een`\ ([=a]"t[=e]n`), a. [AS. eahtat[=y]ne,
   eahtat[=e]ne. See Eight, and Ten, and cf. Eighty.]
   Eight and ten; as, eighteen pounds.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Eighteen \Eight"een`\, n.
   1. The number greater by a unit than seventeen; eighteen
      units or objects.
      [1913 Webster]

   2. A symbol denoting eighteen units, as 18 or xviii.
      [1913 Webster]

WordNet (r) 3.0 (2006):

eighteen
    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]