sinking spell vs decline

sinking spell and decline both are nouns.

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

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

Other hypernyms of sinking spell include diminution.

sinking spell (noun)	decline (noun)
a temporary decline in health or value	a downward slope or bend
	a gradual decrease; as of stored charge or current
	change toward something smaller or lower
	a condition inferior to an earlier condition; a gradual falling off from a better state; decline

sinking spell (verb)	decline (verb)
	grow smaller
	grow worse
	show unwillingness towards
	inflect for number, gender, case, etc.
	go down
	fall in value
	not accept as true