Source : Webster's Revised Unabridged Dictionary (1913)

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.

            A general peace and serenity newly succeeded a
            general trouble.                      --Sir W.
                                                  Temple.

   2. Calmness of mind; eveness of temper; undisturbed state;
      coolness; composure.

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

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

Source : WordNet®

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