decline in quality vs decline

decline in quality and decline both are nouns.

Word	Noun	Adjective	Verb	Adverb
decline in quality	Yes	No	No	No
decline	Yes	No	Yes	No

As nouns, decline is a hypernym of decline in quality; that is, decline is a word with a broader meaning than decline in quality:

Other hypernyms of decline in quality include diminution.

decline in quality (noun)	decline (noun)
process of changing to an inferior state	a downward slope or bend
	a gradual decrease; as of stored charge or current
	change toward something smaller or lower
	a condition inferior to an earlier condition; a gradual falling off from a better state; decline

decline in quality (verb)	decline (verb)
	grow smaller
	grow worse
	show unwillingness towards
	inflect for number, gender, case, etc.
	go down
	fall in value
	not accept as true