rarefaction vs drop-off

rarefaction and drop-off both are nouns.

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

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

Other hypernyms of rarefaction include decrease, lessening.

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