downhill vs decline

downhill and decline both are nouns.

Word	Noun	Adjective	Verb	Adverb
downhill	Yes	Yes	No	Yes
decline	Yes	No	Yes	No

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

Other hypernyms of downhill include declension, declination, declivity, descent, downslope, fall.

downhill (noun)	decline (noun)
a ski race down a trail	a downward slope or bend
the downward slope of a hill	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

downhill (adjective)	decline (adjective)
sloping down rather steeply

downhill (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

downhill (adverb)	decline (adverb)
toward the bottom of a hill
toward a lower or inferior state