fatal accident vs casualty

fatal accident and casualty both are nouns.

Word	Noun	Adjective	Verb	Adverb
fatal accident	Yes	No	No	No
casualty	Yes	No	No	No

As nouns, fatal accident and casualty are synonyms defined as:

fatal accident (noun)	casualty (noun)
an accident that causes someone to die	a decrease of military personnel or equipment
	an accident that causes someone to die
	someone injured or killed in an accident
	someone injured or killed or captured or missing in a military engagement