quietness vs relaxation

quietness and relaxation both are nouns.

Word	Noun	Adjective	Verb	Adverb
quietness	Yes	No	No	No
relaxation	Yes	No	No	No

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

quietness: a state of peace and quiet
relaxation: a feeling of refreshing tranquility and an absence of tension or worry

Other hyponyms of quietness include ataraxis, heartsease, peace, peace of mind, peacefulness, repose, serenity, easiness.

quietness (noun)	relaxation (noun)
the property of making no sound	freedom from activity (work or strain or responsibility)
a state of peace and quiet	the act of making less strict
	a method of solving simultaneous equations by guessing a solution and then reducing the errors that result by successive approximations until all the errors are less than some specified amount
	an occurrence of control or strength weakening
	a feeling of refreshing tranquility and an absence of tension or worry
	(physics) the exponential return of a system to equilibrium after a disturbance
	(physiology) the gradual lengthening of inactive muscle or muscle fibers