The Collaborative International Dictionary of English v.0.48:

Thirteen \Thir"teen`\ (th[~e]r"t[=e]n`), a. [OE. threttene, AS.
   [thorn]re['o]t[=e]ne, [thorn]re['o]tyne. See Three, and
   Ten, and cf. Thirty.]
   One more than twelve; ten and three; as, thirteen ounces or
   pounds.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thirteen \Thir"teen`\, n.
   1. The number greater by one than twelve; the sum of ten and
      three; thirteen units or objects.
      [1913 Webster]

   2. A symbol representing thirteen units, as 13 or xiii.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thirteen
    adj 1: being one more than twelve [syn: thirteen, 13,
           xiii]
    n 1: the cardinal number that is the sum of twelve and one [syn:
         thirteen, 13, XIII, baker's dozen, long dozen]