lessening vs casualty

lessening and casualty both are nouns.

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

As nouns, casualty is a hyponym of lessening; that is, casualty is a word with a more specific, narrower meaning than lessening:

Other hyponyms of lessening include shrinkage, shrinking, sinking, attrition, rarefaction, dwindling, dwindling away, waning.

lessening (noun)	casualty (noun)
a change downward	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