declension vs falloff

declension and falloff both are nouns.

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

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

Other hyponyms of declension include drop-off, falling off, slack, slump.

declension (noun)	falloff (noun)
a class of nouns or pronouns or adjectives in Indo-European languages having the same (or very similar) inflectional forms	a noticeable deterioration in performance or quality
a downward slope or bend
process of changing to an inferior state
the inflection of nouns and pronouns and adjectives in Indo-European languages