drop-off vs declension

drop-off and declension both are nouns.

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

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

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

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