drop-off vs sinking

drop-off and sinking both are nouns.

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

As nouns, sinking is a hyponym of drop-off; that is, sinking is a word with a more specific, narrower meaning than drop-off:

Other hyponyms of drop-off include shrinkage, shrinking, casualty, attrition, rarefaction, dwindling, dwindling away, waning.

drop-off (noun)	sinking (noun)
a change downward	a descent as through liquid (especially through water)
a steep high face of rock	a slow fall or decline (as for lack of strength)
a noticeable deterioration in performance or quality	a feeling caused by uneasiness or apprehension