The Collaborative International Dictionary of English v.0.48:

Constitutive \Con"sti*tu`tive\, a.
   1. Tending or assisting to constitute or compose; elemental;
      essential.
      [1913 Webster]

            An ingredient and constitutive part of every virtue.
                                                  --Barrow.
      [1913 Webster]

   2. Having power to enact, establish, or create; instituting;
      determining. --Sir W. Hamilton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

constitutive
    adj 1: constitutional in the structure of something (especially
           your physical makeup) [syn: constituent(a),
           constitutional, constitutive(a), organic]