casualty vs lessening

casualty and lessening both are nouns.

Word	Noun	Adjective	Verb	Adverb
casualty	Yes	No	No	No
lessening	Yes	No	No	No

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

Other hypernyms of casualty include decrease, drop-off.

casualty (noun)	lessening (noun)
a decrease of military personnel or equipment	a change downward
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