The Collaborative International Dictionary of English v.0.48:

Fifteen \Fif"teen`\, a. [OE. fiftene, AS. f[imac]ft[=y]ne,
   f[imac]ft[=e]ne. See Five, and Ten, and cf. Fifty.]
   Five and ten; one more than fourteen.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fifteen \Fif"teen`\, n.
   1. The sum of five and ten; fifteen units or objects.
      [1913 Webster]

   2. A symbol representing fifteen units, as 15, or xv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fifteen
    adj 1: being one more than fourteen [syn: fifteen, 15, xv]
    n 1: the cardinal number that is the sum of fourteen and one
         [syn: fifteen, 15, XV]