lowering vs diminution

lowering and diminution both are nouns.

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

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

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

lowering (noun)	diminution (noun)
the act of causing to become less	the act of decreasing or reducing something
the act of causing something to move to a lower level	the statement of a theme in notes of lesser duration (usually half the length of the original)
	change toward something smaller or lower

lowering (adjective)	diminution (adjective)
darkened by clouds