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]