declivity vs declension

declivity and declension both are nouns.

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

As nouns, declivity and declension are synonyms defined as:

Other synonyms of declivity include declination, decline, descent, downslope, fall.

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