decrement vs diminution

decrement and diminution both are nouns.

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

As nouns, diminution is a hyponym of decrement; that is, diminution is a word with a more specific, narrower meaning than decrement:

Other hyponyms of decrement include decay, decline, decline, desensitisation, desensitization, narrowing, slippage, wastage.

decrement (noun)	diminution (noun)
the amount by which something decreases	the act of decreasing or reducing something
a process of becoming smaller or shorter	the statement of a theme in notes of lesser duration (usually half the length of the original)
	change toward something smaller or lower