decrease vs lessening

decrease and lessening both are nouns.

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

As nouns, decrease and lessening are synonyms defined as:

Other synonyms of decrease include drop-off.

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

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