Clear, untroubled, sustained inner peace.
Clear, untroubled, sustained inner peace — the deepest and most settled calm, often with a spiritual coloring.
The deepest and most settled member, often lofty or spiritual — primarily an enduring inner state, not a momentary reaction. Where calmness is the bare absence of agitation, serenity implies positive depth and radiance; where tranquility is quiet stillness of a setting or mind, serenity is luminous untroubled peace of the person. Unlike equanimity (evenness under stress) or composure (holding together in a trying moment), serenity is peace in the absence of, or above, turmoil.
Mid-15c., “fair, calm weather,” from Latin serenitas, from serenus “clear, unclouded, calm” (of weather). The deeper PIE root is *ksero- “dry.”
Originally of weather/sky (“fair, calm, clear”), applied to a person's mind by the 1590s — the documented weather-to-mind metaphor. Also once a title of honor for kings, echoing Latin serenitas for emperors and popes. No reliable recent-generation shift.