decline in quality vs declension

decline in quality and declension both are nouns.

Word	Noun	Adjective	Verb	Adverb
decline in quality	Yes	No	No	No
declension	Yes	No	No	No

As nouns, decline in quality and declension are synonyms defined as:

decline in quality and declension: process of changing to an inferior state

Other synonyms of decline in quality include deterioration, worsening.

decline in quality (noun)	declension (noun)
process of changing to an inferior state	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