The Collaborative International Dictionary of English v.0.48:

Nineteen \Nine"teen`\, a. [AS. nigont[=y]ne, nigont[=e]ne. See
   Nine, and Ten.]
   Nine and ten; eighteen and one more; one less than twenty;
   as, nineteen months.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nineteen \Nine"teen`\, n.
   1. The number greater than eighteen by a unit; the sum of ten
      and nine; nineteen units or objects.
      [1913 Webster]

   2. A symbol for nineteen units, as 19 or xix.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nineteen
    adj 1: being one more than eighteen [syn: nineteen, 19,
           xix]
    n 1: the cardinal number that is the sum of eighteen and one
         [syn: nineteen, 19, XIX]