Webster's Revised Unabridged Dictionary · 1913

Serenity

Serenity , noun

[Latin serenuas: compare French sérénité.]

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

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