declension vs deterioration

declension and deterioration both are nouns.

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

As nouns, declension and deterioration are synonyms defined as:

Other synonyms of declension include decline in quality, worsening.

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