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 seat
    n 1: the throne of God
    2: the golden covering of the ark of the covenant