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]