lessening vs decrease

lessening and decrease both are nouns.

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

As nouns, lessening and decrease are synonyms defined as:

Other synonyms of lessening include drop-off.

lessening (noun)	decrease (noun)
a change downward	the act of decreasing or reducing something
	the amount by which something decreases
	a change downward
	a process of becoming smaller or shorter

lessening (verb)	decrease (verb)
	decrease in size, extent, or range
	make become smaller