The Collaborative International Dictionary of English v.0.48:

Calmness \Calm"ness\, n.
   The state of quality of being calm; quietness; tranquillity;
   self-repose.
   [1913 Webster]

         The gentle calmness of the flood.        --Denham.
   [1913 Webster]

         Hes calmness was the repose of conscious power. --E.
                                                  Everett.

   Syn: Quietness; quietude; stillness; tranquillity; serenity;
        repose; composure; sedateness; placidity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

calmness
    n 1: steadiness of mind under stress; "he accepted their
         problems with composure and she with equanimity" [syn:
         composure, calm, calmness, equanimity] [ant:
         discomposure]
    2: an absence of strong winds or rain
    3: a feeling of calm; an absence of agitation or excitement
       [ant: agitation]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "calmness":
   Buddha-like composure, Oriental calm, abnegation, abstinence,
   ataraxia, ataraxy, awelessness, calm, calm disposition,
   calm of mind, composure, conservatism, constraint, contemplation,
   continence, control, cool, cool head, coolheadedness, coolness,
   countenance, dispassion, easy mind, evenness, expectation,
   gentleness, golden mean, happy medium, icy calm, icy nerves,
   impartiality, imperturbability, imperturbation, indisturbance,
   inexcitability, iron nerves, judiciousness, juste-milieu,
   law and order, lenity, lucid stillness, marmoreal repose,
   meden agan, mental composure, middle way, mildness, moderateness,
   moderation, moderationism, nervelessness, nerves of steel,
   neutrality, nirvana, no nerves, nodding acceptance, nonamazedness,
   nonamazement, nonastonishment, nonmarveling, nonviolence,
   nonwonder, nonwondering, nothing in excess, order, orderliness,
   pacifism, peace, peace and quiet, peace of mind, peacefulness,
   philosophic composure, philosophicalness, philosophy, phlegm,
   placidity, placidness, prudence, quiescence, quiescency, quiet,
   quiet life, quiet mind, quietism, quietness, quietude, repose,
   rest, restfulness, restraint, sangfroid, satori, self-abnegation,
   self-control, self-denial, self-restraint, serenity, silence,
   silken repose, sleep, slumber, sobriety, soothingness, stability,
   steadiness, steady nerves, steady-handedness, stillness,
   strong nerves, temperance, temperateness, tranquillity,
   unamazedness, unamazement, unastonishment, unexcessiveness,
   unextravagance, unextremeness, unimpressibleness, unnervousness,
   unruffledness, unshakiness, unsurprise, unsurprisedness,
   untremulousness, via media, wise passiveness, wonderlessness