falloff vs declension

falloff and declension both are nouns.

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

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

Other hypernyms of falloff include decline in quality, deterioration, worsening.

falloff (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