casualty vs fatal accident

casualty and fatal accident both are nouns.

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

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

casualty (noun)	fatal accident (noun)
a decrease of military personnel or equipment	an accident that causes someone to die
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