lower vs lessen

lower is a noun but lessen is not a noun.

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

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

Other hypernyms of lower include decrease, minify.

lower (noun)	lessen (noun)
the lower of two berths

lower (adjective)	lessen (adjective)
the bottom one of two

lower (verb)	lessen (verb)
make lower or quieter	decrease in size, extent, or range
set lower	wear off or die down
cause to drop or sink	make become smaller
move something or somebody to a lower position
look angry or sullen, wrinkle one's forehead, as if to signal disapproval