The Collaborative International Dictionary of English v.0.48:

Mitigative \Mit"i*ga*tive\, a. [L. mitigativus: cf. F.
   mitigatif.]
   Tending to mitigate; alleviating.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mitigative
    adj 1: moderating pain or sorrow by making it easier to bear
           [syn: alleviative, alleviatory, lenitive,
           mitigative, mitigatory, palliative]