quietude vs serenity

quietude and serenity both are nouns.

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

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

Other hyponyms of quietude include ataraxis, heartsease, peace, peace of mind, peacefulness, repose, easiness, relaxation.

quietude (noun)	serenity (noun)
a state of peace and quiet	a disposition free from stress or emotion
	the absence of mental stress or anxiety