shortening vs diminution

shortening and diminution both are nouns.

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

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

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

shortening (noun)	diminution (noun)
act of decreasing in length	the act of decreasing or reducing something
fat such as butter or lard used in baked goods	the statement of a theme in notes of lesser duration (usually half the length of the original)
	change toward something smaller or lower