waning vs drop-off

waning and drop-off both are nouns.

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

As nouns, drop-off is a hypernym of waning; that is, drop-off is a word with a broader meaning than waning:

Other hypernyms of waning include decrease, lessening.

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

waning (adjective)	drop-off (adjective)
(of the Moon) pertaining to the period during which the visible surface of the moon decreases