The Collaborative International Dictionary of English v.0.48:

Mercy \Mer"cy\ (m[~e]r"s[y^]), n.; pl. Mercies. [OE. merci, F.
   merci, L. merces, mercedis, hire, pay, reward, LL., equiv. to
   misericordia pity, mercy. L. merces is probably akin to
   merere to deserve, acquire. See Merit, and cf. Amerce.]
   1. Forbearance to inflict harm under circumstances of
      provocation, when one has the power to inflict it;
      compassionate treatment of an offender or adversary;
      clemency.
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            Examples of justice must be made for terror to some;
            examples of mercy for comfort to others. --Bacon.
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   2. Compassionate treatment of the unfortunate and helpless;
      sometimes, favor, beneficence. --Luke x. 37.
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   3. Disposition to exercise compassion or favor; pity;
      compassion; willingness to spare or to help.
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            In whom mercy lacketh and is not founden. --Sir T.
                                                  Elyot.
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   4. A blessing regarded as a manifestation of compassion or
      favor.
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            The Father of mercies and the God of all comfort.
                                                  --2 Cor. i. 3.
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   Mercy seat (Bib.), the golden cover or lid of the Ark of
      the Covenant. See Ark, 2.

   Sisters of Mercy (R. C. Ch.),a religious order founded in
      Dublin in the year 1827. Communities of the same name have
      since been established in various American cities. The
      duties of those belonging to the order are, to attend
      lying-in hospitals, to superintend the education of girls,
      and protect decent women out of employment, to visit
      prisoners and the sick, and to attend persons condemned to
      death.

   To be at the mercy of, to be wholly in the power of.
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   Syn: See Grace.
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WordNet (r) 3.0 (2006):

mercy
    n 1: leniency and compassion shown toward offenders by a person
         or agency charged with administering justice; "he threw
         himself on the mercy of the court" [syn: clemency,
         mercifulness, mercy]
    2: a disposition to be kind and forgiving; "in those days a wife
       had to depend on the mercifulness of her husband" [syn:
       mercifulness, mercy] [ant: mercilessness,
       unmercifulness]
    3: the feeling that motivates compassion [syn: mercifulness,
       mercy]
    4: something for which to be thankful; "it was a mercy we got
       out alive"
    5: alleviation of distress; showing great kindness toward the
       distressed; "distributing food and clothing to the flood
       victims was an act of mercy"

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "mercy":
   acceptance, act of grace, act of kindness, benefaction,
   beneficence, benefit, benevolence, benignancy, benignity, blessing,
   caritas, charity, clemency, clementness, commiseration, compassion,
   condolence, consideration, courtesy, easiness, easygoingness,
   favor, feeling, forbearance, forbearing, forgiveness, generosity,
   gentleness, good deed, good offices, good turn, goodwill, grace,
   graciousness, humaneness, humanity, indulgence, kind deed,
   kind offices, kindliness, kindly act, kindness, labor of love,
   laxness, lenience, leniency, lenientness, lenity, liberality,
   magnanimity, mercifulness, mildness, mitigation, mitzvah,
   moderateness, obligation, office, pardon, pathos, patience, pity,
   quarter, relief, reprieve, ruth, self-pity, service, softness,
   sympathy, tenderness, thoughtfulness, tolerance, turn



Easton's 1897 Bible Dictionary:

Mercy
   compassion for the miserable. Its object is misery. By the
   atoning sacrifice of Christ a way is open for the exercise of
   mercy towards the sons of men, in harmony with the demands of
   truth and righteousness (Gen. 19:19; Ex. 20:6; 34:6, 7; Ps.
   85:10; 86:15, 16). In Christ mercy and truth meet together.
   Mercy is also a Christian grace (Matt. 5:7; 18:33-35).


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

MERCY, crim. law. The total or partial remission of a punishment to which a
convict is subject. When the whole punishment is remitted, it is called a
pardon; (q.v.) when only a part of the punishment is remitted, it is
frequently a conditional pardon; or before sentence, it is called clemency
or mercy. Vide Rutherf. Inst. 224; 1 Kent, Com. 265; 3 Story, Const. Sec.
1488.



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

MERCY, Practice. To be in mercy, signifies to be liable to punishment at the
discretion of the judge.



The Devil's Dictionary (1881-1906):

MERCY, n.  An attribute beloved of detected offenders.