price reduction vs diminution

price reduction and diminution both are nouns.

Word	Noun	Adjective	Verb	Adverb
price reduction	Yes	No	No	No
diminution	Yes	No	No	No

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

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

price reduction (noun)	diminution (noun)
the act of reducing the selling price of merchandise	the act of decreasing or reducing something
	the statement of a theme in notes of lesser duration (usually half the length of the original)
	change toward something smaller or lower