The Collaborative International Dictionary of English v.0.48:

Ameliorative \A*mel"io*ra*tive\, a.
   Tending to ameliorate; producing amelioration or improvement;
   as, ameliorative remedies, efforts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ameliorative
    adj 1: tending to ameliorate [syn: ameliorating(a),
           ameliorative, amelioratory, meliorative]