lessening vs sinking

lessening and sinking both are nouns.

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

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

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

lessening (noun)	sinking (noun)
a change downward	a descent as through liquid (especially through water)
	a slow fall or decline (as for lack of strength)
	a feeling caused by uneasiness or apprehension