decline vs lessen

decline is a noun but lessen is not a noun.

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

As verbs, lessen is a hypernym of decline; that is, lessen is a word with a broader meaning than decline:

Other hypernyms of decline include decrease, diminish, fall.

decline (noun)	lessen (noun)
a downward slope or bend
a gradual decrease; as of stored charge or current
change toward something smaller or lower
a condition inferior to an earlier condition; a gradual falling off from a better state; decline

decline (verb)	lessen (verb)
grow smaller	decrease in size, extent, or range
grow worse	wear off or die down
show unwillingness towards	make become smaller
inflect for number, gender, case, etc.
go down
fall in value
not accept as true