calmness vs serenity

calmness and serenity both are nouns.

Word	Noun	Adjective	Verb	Adverb
calmness	Yes	No	No	No
serenity	Yes	No	No	No

As nouns, serenity is a hyponym of calmness; that is, serenity is a word with a more specific, narrower meaning than calmness:

Other hyponyms of calmness include aplomb, assuredness, cool, poise, sang-froid, placidity, quiet, repose, tranquility, tranquillity.

calmness (noun)	serenity (noun)
steadiness of mind under stress	a disposition free from stress or emotion
a feeling of calm; an absence of agitation or excitement	the absence of mental stress or anxiety
an absence of strong winds or rain