The Collaborative International Dictionary of English v.0.48:

Peaceful \Peace"ful\, a.
   1. Possessing or enjoying peace; not disturbed by war,
      tumult, agitation, anxiety, or commotion; quiet; tranquil;
      as, a peaceful time; a peaceful country; a peaceful end.
      [1913 Webster]

   2. Not disposed or tending to war, tumult or agitation;
      pacific; mild; calm; peaceable; as, peaceful words.
      [1913 Webster]

   Syn: See Peaceable.
        [1913 Webster] --Peace"ful*ly, adv.. --
        Peace"ful*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

peacefulness
    n 1: a state that is calm and tranquil [syn: peaceableness,
         peacefulness]
    2: the absence of mental stress or anxiety [syn: peace,
       peacefulness, peace of mind, repose, serenity,
       heartsease, ataraxis]