Etymology
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serenity (n.)

mid-15c., serenite, "fair, calm, clear weather," from Old French (Modern French sérénité) and directly from Latin serenitatem (nominative serenitas) "clearness, serenity," from serenus (see serene). Of persons, "tranquility of mind or temper," by 1590s. Also formerly used as a title of honor for kings (mid-15c.), probably from the similar use of Latin serenitas, which was applied to Roman emperors and later to popes.

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Definitions of serenity

serenity (n.)
a disposition free from stress or emotion;
Synonyms: repose / quiet / placidity / tranquillity / tranquility
serenity (n.)
the absence of mental stress or anxiety;
Synonyms: peace / peacefulness / peace of mind / repose / heartsease / ataraxis
From wordnet.princeton.edu