The Collaborative International Dictionary of English v.0.48:

Mitigation \Mit`i*ga"tion\, n. [OE. mitigacioun, F. mitigation,
   fr. L. mitigatio.]
   The act of mitigating, or the state of being mitigated;
   abatement or diminution of anything painful, harsh, severe,
   afflictive, or calamitous; as, the mitigation of pain, grief,
   rigor, severity, punishment, or penalty.
   [1913 Webster]

   Syn: Alleviation; abatement; relief.
        [1913 Webster]

WordNet (r) 3.0 (2006):

mitigation
    n 1: to act in such a way as to cause an offense to seem less
         serious [syn: extenuation, mitigation, palliation]
    2: a partial excuse to mitigate censure; an attempt to represent
       an offense as less serious than it appears by showing
       mitigating circumstances [syn: extenuation, mitigation]
    3: the action of lessening in severity or intensity; "the object
       being control or moderation of economic depressions" [syn:
       moderation, mitigation]

Moby Thesaurus II by Grady Ward, 1.0:

175 Moby Thesaurus words for "mitigation":
   abatement, about-face, abridgment, accommodation, adaptation,
   adjustment, allayment, alleviation, allowance, alteration,
   amelioration, analgesia, anesthesia, anesthetizing, apostasy,
   appeasement, assuagement, attenuation, attrition, benevolence,
   betterment, blunting, break, calming, change, change of heart,
   changeableness, clemency, color, commiseration, compassion,
   condolence, constructive change, continuity, contraction,
   conversion, dampening, damping, deadening, debilitation,
   decontamination, decrease, decrement, decrescence, deduction,
   defection, deflation, degeneration, degenerative change, demulsion,
   depreciation, depression, deterioration, deviation, devitalization,
   difference, dilution, diminishment, diminution, discontinuity,
   divergence, diversification, diversion, diversity, dulcification,
   dulling, dying, dying off, ease, easement, easing, effemination,
   enervation, enfeeblement, evisceration, exhaustion,
   extenuating circumstances, extenuation, extenuative, fade-out,
   falling-off, fatigue, favor, feeling, fitting, flip-flop,
   forbearance, forgiveness, gilding, gloss, grace, gradual change,
   humanity, hushing, improvement, inanition, kindness, languishment,
   leniency, lessening, letdown, letup, lightening, loosening,
   lowering, lulling, melioration, mercy, miniaturization,
   modification, modulation, mollification, numbing, overthrow,
   pacification, palliation, palliative, pardon, pathos, pity,
   qualification, quarter, quietening, quieting, radical change,
   re-creation, realignment, redesign, reduction, reform, reformation,
   relaxation, relief, remaking, remedy, remission, renewal, reprieve,
   reshaping, restructuring, reversal, revival, revivification,
   revolution, ruth, sagging, salving, scaling down, self-pity, shift,
   simplicity, slackening, softening, soothing, subduement,
   subtraction, sudden change, switch, sympathy, tempering, thinning,
   total change, tranquilization, transition, turn, turnabout,
   upheaval, variation, variety, varnish, violent change, weakening,
   whitewash, whitewashing, worsening



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MITIGATION. To make less rigorous or penal.
     2. Crimes are frequently committed under circumstances which are not
justifiable nor excusable, yet they show that the offender has been greatly
tempted; as, for example, when a starving man steals bread to satisfy his
hunger, this circumstance is taken into consideration in mitigation of his
sentence.
     3. In actions for damages, or for torts, matters are frequently proved
in mitigation of damages. In an action for criminal conversation with the
plaintiff's wife, for example, evidence may be given of the wife's general
bad character for want of chastity; or of particular acts of adultery
committed by her, before she became acquainted with the defendant; 12 Mod.
R. 232; Bull. N. P. 27, 296; Selw. N. P. 25; 1 Johns. Cas, 16: or that the
plaintiff has carried on a criminal conversation with other women; Bull. N.
P. 27; or that the plaintiff's wife has made the first advances to the
defendant, 2 Esp. N. P. C. 562; Selw. N. P. 25. See 3 Am. Jur. 287, 313;
Bouv. Inst. Index, h.t.
     4. In actions for libel, although the defendant cannot under the
general issue prove the crime, which is imputed to the plaintiff, yet he is
in many cases allowed to give evidence of the plaintiff's general character
in mitigation of damages. 2 Campb. R. 251; 1 M. & S. 284.