The Collaborative International Dictionary of English v.0.48:

Serenity \Se*ren"i*ty\, n. [L. serenuas: cf. F.
   s['e]r['e]nit['e].]
   1. The quality or state of being serene; clearness and
      calmness; quietness; stillness; peace.
      [1913 Webster]

            A general peace and serenity newly succeeded a
            general trouble.                      --Sir W.
                                                  Temple.
      [1913 Webster]

   2. Calmness of mind; eveness of temper; undisturbed state;
      coolness; composure.
      [1913 Webster]

            I can not see how any men should ever transgress
            those moral rules with confidence and serenity.
                                                  --Locke.
      [1913 Webster]

   Note: Serenity is given as a title to the members of certain
         princely families in Europe; as, Your Serenity.
         [1913 Webster]

WordNet (r) 3.0 (2006):

serenity
    n 1: a disposition free from stress or emotion [syn: repose,
         quiet, placidity, serenity, tranquillity,
         tranquility]
    2: the absence of mental stress or anxiety [syn: peace,
       peacefulness, peace of mind, repose, serenity,
       heartsease, ataraxis]