deterioration vs declension

deterioration and declension both are nouns.

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

As nouns, deterioration and declension are synonyms defined as:

Other synonyms of deterioration include decline in quality, worsening.

deterioration (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 symptom of reduced quality or strength	a downward slope or bend
	process of changing to an inferior state
	the inflection of nouns and pronouns and adjectives in Indo-European languages