decline vs declension

decline and declension both are nouns.

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

As nouns, decline and declension are synonyms defined as:

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

As nouns, declension is a hyponym of decline; that is, declension is a word with a more specific, narrower meaning than decline:

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

decline (verb)	declension (verb)
grow smaller
grow worse
show unwillingness towards
inflect for number, gender, case, etc.
go down
fall in value
not accept as true