means Calm, untroubled, and peacefully composed, whether describing a person, a sky, or a stretch of water.

from From Latin 'serenus,' meaning clear and unclouded — originally said of weather, a bright fair sky with no storm in it. It drifted into English in the 1500s, carrying that meteorological calm with it, which is why a serene face still sounds a little like good weather. The royal title 'Serene Highness' comes from the same root, the idea being a ruler as cloudlessly composed as a fine day.