The Collaborative International Dictionary of English v.0.48:

mitigating \mitigating\ adj.
   serving to reduce blame; -- of situations; as, mitigating
   factors; mitigating circumstances. Opposite of aggravating.
   [Narrower terms: exculpatory]

   Syn: extenuating.
        [WordNet 1.5 +PJC]

The Collaborative International Dictionary of English v.0.48:

Mitigate \Mit"i*gate\, v. t. [imp. & p. p. Mitigated; p. pr. &
   vb. n. Mitigating.] [L. mitigatus, p. p. of mitigare to
   soften, mitigate; mitis mild, soft + the root of agere to do,
   drive.]
   1. To make less severe, intense, harsh, rigorous, painful,
      etc.; to soften; to meliorate; to alleviate; to diminish;
      to lessen; as, to mitigate heat or cold; to mitigate
      grief.
      [1913 Webster]

   2. To make mild and accessible; to mollify; -- applied to
      persons. [Obs.]
      [1913 Webster]

            This opinion . . . mitigated kings into companions.
                                                  --Burke.
      [1913 Webster]

   Syn: To alleviate; assuage; allay. See Alleviate.
        [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "mitigating":
   abating, allaying, alleviating, alleviative, altering, analgesic,
   anesthetic, anodyne, assuaging, assuasive, balmy, balsamic,
   benumbing, blunting, bounding, cathartic, chastening, cleansing,
   cushioning, dampening, damping, deadening, demulcent, diminishing,
   dulling, easing, emollient, excusatory, extenuating, extenuatory,
   justifying, lenitive, lessening, limitative, limiting, mitigative,
   mitigatory, modificatory, modifying, modulatory, numbing,
   pain-killing, palliative, purgative, qualificative, qualificatory,
   qualifying, reducing, relaxing, relieving, remedial, restricting,
   restrictive, softening, soothing, subduing, tempering,
   vindicating