drop off vs decrease

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

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

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

Other hypernyms of drop off include diminish, fall, lessen.

drop off (noun)	decrease (noun)
	the act of decreasing or reducing something
	the amount by which something decreases
	a change downward
	a process of becoming smaller or shorter

drop off (verb)	decrease (verb)
change from a waking to a sleeping state	decrease in size, extent, or range
fall to a lower standard	make become smaller
fall or diminish
retreat
remove (cargo, people, etc.) from and leave