calm, peaceful, or tranquil; unruffled

a serene landscape; serene old age.

clear; fair

serene weather.

( usually initial capital letter ) most high or august (used as a royal epithet, usually prec. by his, your, etc.)

His Serene Highness.

serenity; tranquillity.

Archaic . a clear or tranquil expanse of sea or sky.