The Collaborative International Dictionary of English v.0.48:

Eighteenth \Eight"eenth`\, a. [From Eighteen.]
   1. Next in order after the seventeenth.
      [1913 Webster]

   2. Consisting of one of eighteen equal parts or divisions of
      a thing.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Eighteenth \Eight"eenth`\, n.
   1. The quotient of a unit divided by eighteen; one of
      eighteen equal parts or divisions.
      [1913 Webster]

   2. The eighth after the tenth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

eighteenth
    adj 1: coming next after the seventeenth in position [syn:
           eighteenth, 18th]
    n 1: position 18 in a countable series of things