declension vs decline

declension and decline both are nouns.

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

As nouns, declension and decline are synonyms defined as:

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

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

Other hypernyms of declension include diminution.

declension (noun)	decline (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	a gradual decrease; as of stored charge or current
process of changing to an inferior state	change toward something smaller or lower
the inflection of nouns and pronouns and adjectives in Indo-European languages	a condition inferior to an earlier condition; a gradual falling off from a better state; decline

declension (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