The Collaborative International Dictionary of English v.0.48:

Twelve \Twelve\, a. [OE. twelve, twelf, AS. twelf; akin to
   OFries. twelf, twelef, twilif, OS. twelif, D. twaalf, G.
   zw["o]lf, OHG. zwelif, Icel. t[=o]lf, Sw. tolf, Dan. tolv,
   Goth. twalif, from the root of E. two + the same element as
   in the second part of E. eleven. See Two, and Eleven.]
   One more that eleven; two and ten; twice six; a dozen.
   [1913 Webster]

   Twelve-men's morris. See the Note under Morris.

   Twelve Tables. (Rom. Antiq.) See under Table.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twelve \Twelve\, n.
   1. The number next following eleven; the sum of ten and two,
      or of twice six; twelve units or objects; a dozen.
      [1913 Webster]

   2. A symbol representing twelve units, as 12, or xii.
      [1913 Webster]

   The Twelve (Script.), the twelve apostles. --Matt. xxvi.
      20.
      [1913 Webster]

WordNet (r) 3.0 (2006):

twelve
    adj 1: denoting a quantity consisting of 12 items or units [syn:
           twelve, 12, xii, dozen]
    n 1: the cardinal number that is the sum of eleven and one [syn:
         twelve, 12, XII, dozen]