From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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]