falling off vs declension

falling off and declension both are nouns.

Word	Noun	Adjective	Verb	Adverb
falling off	Yes	No	No	No
declension	Yes	No	No	No

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

Other hypernyms of falling off include decline in quality, deterioration, worsening.

falling off (noun)	declension (noun)
a noticeable deterioration in performance or quality	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