decline vs diminish

decline is a noun but diminish is not a noun.

Word	Noun	Adjective	Verb	Adverb
decline	Yes	No	Yes	No
diminish	No	No	Yes	No

As verbs, diminish is a hypernym of decline; that is, diminish is a word with a broader meaning than decline:

Other hypernyms of decline include decrease, fall, lessen.

decline (noun)	diminish (noun)
a downward slope or bend
a gradual decrease; as of stored charge or current
change toward something smaller or lower
a condition inferior to an earlier condition; a gradual falling off from a better state; decline

decline (verb)	diminish (verb)
grow smaller	decrease in size, extent, or range
grow worse	lessen the authority, dignity, or reputation of
show unwillingness towards
inflect for number, gender, case, etc.
go down
fall in value
not accept as true