subtraction vs diminution

subtraction and diminution both are nouns.

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

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

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

subtraction (noun)	diminution (noun)
the act of subtracting (removing a part from the whole)	the act of decreasing or reducing something
an arithmetic operation in which the difference between two numbers is calculated	the statement of a theme in notes of lesser duration (usually half the length of the original)
	change toward something smaller or lower