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

WordNet (r) 3.0 (2006):

mitigate
    v 1: lessen or to try to lessen the seriousness or extent of;
         "The circumstances extenuate the crime" [syn: extenuate,
         palliate, mitigate]
    2: make less severe or harsh; "mitigating circumstances"