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]

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"

Moby Thesaurus II by Grady Ward, 1.0:

206 Moby Thesaurus words for "mitigate":
   abate, accommodate, adapt, adjust, adjust to, allay, alleviate,
   allow for, alter, ameliorate, anesthetize, appease, assuage,
   attemper, attenuate, bank the fire, bate, be changed,
   be converted into, be renewed, benumb, better, blunt, bottom out,
   box in, break, break up, calm, change, chasten, checker, chop,
   chop and change, circumscribe, color, come about, come around,
   come round, condition, constrain, control, convert, cramp, cripple,
   cushion, damp, dampen, de-emphasize, deaden, deaden the pain,
   debilitate, decrease, deform, degenerate, denature, deteriorate,
   deviate, devitalize, dilute, diminish, diverge, diversify,
   downplay, dull, ease, ease matters, ease off, ease up, enervate,
   enfeeble, eviscerate, exhaust, extenuate, fit, flop, foment,
   give relief, gloss over, gruel, haul around, hedge, hedge about,
   ignore, improve, jibe, keep within bounds, lay, lay low, leaven,
   lenify, lessen, let down, let up, let up on, lighten, limit, loose,
   loosen, lull, make allowance for, meliorate, mince, moderate,
   modify, modulate, mollify, mutate, narrow, numb, obtund, overthrow,
   pad, palliate, placate, play down, poultice, pour balm into,
   pour oil on, qualify, rattle, re-create, realign, rebuild,
   reconstruct, redesign, reduce, reduce the temperature, refit,
   reform, regulate by, relax, relieve, remake, remit, renew, reshape,
   restrain, restrict, restructure, revamp, revive, ring the changes,
   salve, sap, season, set conditions, set limits, shake, shake up,
   shift, shift the scene, shuffle the cards, slack, slack off,
   slack up, slacken, slake, slow down, slur over, smother, sober,
   sober down, soft-pedal, soften, soften up, soothe, stifle, still,
   stupe, subdue, subvert, suppress, swerve, tack, take a turn, tame,
   temper, tone down, tune down, turn, turn aside, turn into,
   turn the corner, turn the scale, turn the tables, turn the tide,
   turn upside down, unbend, unbrace, undergo a change, undermine,
   underplay, unman, unnerve, unstrain, unstrengthen, unstring,
   varnish, vary, veer, warp, water down, weaken, whitewash,
   work a change, worsen