declension vs declivity

declension and declivity both are nouns.

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

As nouns, declension and declivity are synonyms defined as:

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

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