drop off vs decline

drop off is not a noun while decline is a noun.

Word	Noun	Adjective	Verb	Adverb
drop off	No	No	Yes	No
decline	Yes	No	Yes	No

As verbs, decline is a hypernym of drop off; that is, decline is a word with a broader meaning than drop off:

Other hypernyms of drop off include worsen.

drop off (noun)	decline (noun)
	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

drop off (verb)	decline (verb)
change from a waking to a sleeping state	grow smaller
fall to a lower standard	grow worse
fall or diminish	show unwillingness towards
retreat	inflect for number, gender, case, etc.
remove (cargo, people, etc.) from and leave	go down
	fall in value
	not accept as true