lessening vs waning

lessening and waning both are nouns.

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

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

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

lessening (noun)	waning (noun)
a change downward	a gradual decrease in magnitude or extent

lessening (adjective)	waning (adjective)
	(of the Moon) pertaining to the period during which the visible surface of the moon decreases