downhill vs declination

downhill and declination both are nouns.

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

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

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

downhill (noun)	declination (noun)
a ski race down a trail	a polite refusal of an invitation
the downward slope of a hill	(astronomy) the angular distance of a celestial body north or to the south of the celestial equator; expressed in degrees; used with right ascension to specify positions on the celestial sphere
	a downward slope or bend
	a condition inferior to an earlier condition; a gradual falling off from a better state; decline

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

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