Merriam-Webster's Collegiate® Dictionary, 11th Edition
SERENE
I. adjective
Etymology: Middle English, from Latin serenus clear, cloudless, untroubled
Date: 15th century
1. clear and free of storms or unpleasant change , shining bright and steady , august , marked by or suggestive of utter calm and unruffled repose or quietude , see: calm serenely adverb sereneness noun
II. noun
Date: 1644
a serene condition or expanse (as of sky, sea, or light), serenity , tranquility