weakening vs diminution

weakening and diminution both are nouns.

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

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

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

weakening (noun)	diminution (noun)
the act of reducing the strength of something	the act of decreasing or reducing something
becoming weaker	the statement of a theme in notes of lesser duration (usually half the length of the original)
	change toward something smaller or lower

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