weaken vs decrease

weaken is not a noun while decrease is a noun.

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

As verbs, decrease is a hypernym of weaken; that is, decrease is a word with a broader meaning than weaken:

Other hypernyms of weaken include diminish, fall, lessen.

As verbs, decrease is a hypernym of weaken; that is, decrease is a word with a broader meaning than weaken:

Other hypernyms of weaken include lessen, minify.

weaken (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

weaken (verb)	decrease (verb)
become weaker	decrease in size, extent, or range
lessen the strength of	make become smaller
reduce the level or intensity or size or scope of
lessen in force or effect
destroy property or hinder normal operations