drop off vs drop-off

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

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

drop off (noun)	drop-off (noun)
	a change downward
	a steep high face of rock
	a noticeable deterioration in performance or quality

drop off (verb)	drop-off (verb)
change from a waking to a sleeping state
fall to a lower standard
fall or diminish
retreat
remove (cargo, people, etc.) from and leave