Source : Webster's Revised Unabridged Dictionary (1913)

Eighteen \Eight"een`\, a. [AS. eahtat?ne, eahtat?ne. See
   {Eight}, and {Ten}, and cf. {Eighty}.]
   Eight and ten; as, eighteen pounds.

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

   2. A symbol denoting eighteen units, as 18 or xviii.

Source : WordNet®

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