The Collaborative International Dictionary of English v.0.48:

Nineteenth \Nine"teenth`\, n.
   1. The quotient of a unit divided by nineteen; one of
      nineteen equal parts of anything.
      [1913 Webster]

   2. The next in order after the eighteenth.
      [1913 Webster]

   3. (Mus.) An interval of two octaves and a fifth.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nineteenth \Nine"teenth`\, a. [Cf. AS. nigonte['o][eth]a.]
   1. Following the eighteenth and preceding the twentieth;
      coming after eighteen others.
      [1913 Webster]

   2. Constituting or being one of nineteen equal parts into
      which anything is divided.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nineteenth
    adj 1: coming next after the eighteenth in position [syn:
           nineteenth, 19th]
    n 1: position 19 in a countable series of things