drop-off vs rarefaction

drop-off and rarefaction both are nouns.

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

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

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

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