the state or quality of being serene, calm, or tranquil; sereneness.
2.
( usually initial capital letter ) a title of honor, respect, or reverence, used in speaking of or to certain members of royalty (usually preceded by his, your, etc.).
Origin: 1400–50;late Middle Englishserenite < Latinserēnitās. See serene, -ity
1530s, of weather, 1590s, of persons," from Fr. sérénité, from L. serenitatem (nom. serenitas) "clearness, serenity," from serenus (see serene). Earliest use (mid-15c.) was as a title of honor for kings.