declension vs drop-off

declension and drop-off both are nouns.

Word	Noun	Adjective	Verb	Adverb
declension	Yes	No	No	No
drop-off	Yes	No	No	No

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

Other hyponyms of declension include falling off, falloff, slack, slump.

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