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.
      [1913 Webster]

            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.
      [1913 Webster]

   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):

mercifulness
    n 1: the feeling that motivates compassion [syn: 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: 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]