The Collaborative International Dictionary of English v.0.48:

Peaceable \Peace"a*ble\, a. [OE. peisible, F. paisible.]
   Begin in or at peace; tranquil; quiet; free from, or not
   disposed to, war, disorder, or excitement; not quarrelsome.
   -- Peace"a*ble*ness, n. -- Peace"a*bly, adv.
   [1913 Webster]

   Syn: Peaceful; pacific; tranquil; quiet; mild; undisturbed;
        serene; still.

   Usage: Peaceable, Peaceful. Peaceable describes the state
          of an individual, nation, etc., in reference to
          external hostility, attack, etc.; peaceful, in respect
          to internal disturbance. The former denotes "in the
          spirit of peace;" latter; "in the possession or
          enjoyment of peace." A peaceable adjustment of
          difficulties; a peaceful life, scene.
          [1913 Webster]

WordNet (r) 3.0 (2006):

peaceableness
    n 1: a state that is calm and tranquil [syn: peaceableness,
         peacefulness]