The Collaborative International Dictionary of English v.0.48:

Merciful \Mer"ci*ful\, a. [Mercy + -ful.]
   1. Full of mercy; having or exercising mercy; disposed to
      pity and spare offenders; unwilling to punish. Opposite of
      merciless.
      [1913 Webster]

            The Lord, the Lord God, merciful and gracious. --Ex.
                                                  xxxiv. 6.
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            Be merciful, great duke, to men of mold. --Shak.
      [1913 Webster]

   2. Unwilling to give pain; compassionate.
      [1913 Webster]

            A merciful man will be merciful to his beast. --Old
                                                  Proverb.
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   Syn: Compassionate; tender; humane; gracious; kind; mild;
        clement; benignant.
        [1913 Webster] -- Mer"ci*ful*ly, adv. --
        Mer"ci*ful*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

merciful
    adj 1: showing or giving mercy; "sought merciful treatment for
           the captives"; "a merciful god" [ant: merciless,
           unmerciful]
    2: (used conventionally of royalty and high nobility) gracious;
       "our merciful king"