downhill vs declension

downhill and declension both are nouns.

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

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

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

downhill (noun)	declension (noun)
a ski race down a trail	a class of nouns or pronouns or adjectives in Indo-European languages having the same (or very similar) inflectional forms
the downward slope of a hill	a downward slope or bend
	process of changing to an inferior state
	the inflection of nouns and pronouns and adjectives in Indo-European languages

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

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