The Collaborative International Dictionary of English v.0.48:

Duodecimal \Du`o*dec"i*mal\, a. [L. duodecim twelve. See
   Dozen.]
   Proceeding in computation by twelves; expressed in the scale
   of twelves. -- Du`o*dec"i*mal*ly, adv.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Duodecimal \Du`o*dec"i*mal\, n.
   1. A twelfth part; as, the duodecimals of an inch.
      [1913 Webster]

   2. pl. (Arch.) A system of numbers, whose denominations rise
      in a scale of twelves, as of feet and inches. The system
      is used chiefly by artificers in computing the superficial
      and solid contents of their work.
      [1913 Webster]

WordNet (r) 3.0 (2006):

duodecimal
    adj 1: based on twelve; "the duodecimal number system"
    n 1: one part in twelve equal parts [syn: one-twelfth,
         twelfth, twelfth part, duodecimal]