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]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "peacefulness":
   Buddha-like composure, Oriental calm, ataraxia, ataraxy,
   calm disposition, calm of mind, calmness, commodiousness,
   composure, contemplation, convenience, cool, coolheadedness,
   coolness, countenance, coziness, cushioniness, easiness, easy mind,
   friendliness, homelikeness, homeliness, homeyness, hospitality,
   icy calm, imperturbability, imperturbation, indisturbance,
   law and order, lucid stillness, luxuriousness, marmoreal repose,
   mental composure, nirvana, order, orderliness, peace,
   peace and quiet, peace of mind, philosophic composure,
   philosophicalness, philosophy, placidity, placidness, quiescence,
   quiescency, quiet, quiet life, quiet mind, quietism, quietness,
   quietude, repose, reposefulness, rest, restfulness, roominess,
   sangfroid, satori, serenity, silence, silken repose, sleep,
   slumber, snugness, softness, soothingness, stillness, tranquillity,
   unruffledness, warmness, wise passiveness