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]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "serenity":
   Buddha-like composure, Oriental calm, abnegation, abstinence,
   aplomb, ataraxia, ataraxy, calm, calm disposition, calm of mind,
   calmness, composure, conservatism, constraint, contemplation,
   continence, control, cool, coolheadedness, coolness, countenance,
   dispassion, easy mind, evenness, gentleness, golden mean,
   happy medium, icy calm, impartiality, imperturbability,
   imperturbation, indisturbance, judiciousness, juste-milieu,
   law and order, lenity, lucid stillness, marmoreal repose,
   meden agan, mental composure, middle way, mildness, moderateness,
   moderation, moderationism, neutrality, nirvana, nonchalance,
   nonviolence, nothing in excess, order, orderliness, pacifism,
   peace, peace and quiet, peace of mind, peaceableness, peacefulness,
   philosophic composure, philosophicalness, philosophy, placidity,
   placidness, poise, prudence, quiescence, quiescency, quiet,
   quiet life, quiet mind, quietism, quietness, quietude, repose,
   rest, restfulness, restraint, sangfroid, satori, self-abnegation,
   self-control, self-denial, self-possession, self-restraint,
   silence, silken repose, sleep, slumber, sobriety, soothingness,
   stability, steadiness, stillness, temperance, temperateness,
   tranquillity, unexcessiveness, unexcitability, unextravagance,
   unextremeness, unruffledness, via media, wise passiveness