sinking spell vs diminution

sinking spell and diminution both are nouns.

Word	Noun	Adjective	Verb	Adverb
sinking spell	Yes	No	No	No
diminution	Yes	No	No	No

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

Other hypernyms of sinking spell include decline.

sinking spell (noun)	diminution (noun)
a temporary decline in health or value	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