declension vs diminution

declension and diminution both are nouns.

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

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

Other hypernyms of declension include decline.

declension (noun)	diminution (noun)
a class of nouns or pronouns or adjectives in Indo-European languages having the same (or very similar) inflectional forms	the act of decreasing or reducing something
a downward slope or bend	the statement of a theme in notes of lesser duration (usually half the length of the original)
process of changing to an inferior state	change toward something smaller or lower
the inflection of nouns and pronouns and adjectives in Indo-European languages