lessen vs decrease

lessen is not a noun while decrease is a noun.

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

As verbs, lessen and decrease are synonyms defined as:

Other synonyms of lessen include diminish, fall.

As verbs, lessen and decrease are synonyms defined as:

Other synonyms of lessen include minify.

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

lessen (verb)	decrease (verb)
decrease in size, extent, or range	decrease in size, extent, or range
wear off or die down	make become smaller
make become smaller