weakening vs decrease

weakening and decrease both are nouns.

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

As nouns, decrease is a hypernym of weakening; that is, decrease is a word with a broader meaning than weakening:

Other hypernyms of weakening include diminution, reduction, step-down.

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

weakening (adjective)	decrease (adjective)
causing debilitation
moderating by making pain or sorrow weaker

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