casualty vs drop-off

casualty and drop-off both are nouns.

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

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

Other hypernyms of casualty include decrease, lessening.

casualty (noun)	drop-off (noun)
a decrease of military personnel or equipment	a change downward
an accident that causes someone to die	a steep high face of rock
someone injured or killed in an accident	a noticeable deterioration in performance or quality
someone injured or killed or captured or missing in a military engagement