detumescence vs diminution

detumescence and diminution both are nouns.

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

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

Other hypernyms of detumescence include decline.

detumescence (noun)	diminution (noun)
diminution of swelling; the subsidence of anything swollen	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