decrease vs diminution

decrease and diminution both are nouns.

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

As nouns, decrease and diminution are synonyms defined as:

Other synonyms of decrease include reduction, step-down.

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

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

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

decrease (verb)	diminution (verb)
decrease in size, extent, or range
make become smaller