declassification vs diminution

declassification and diminution both are nouns.

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

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

declassification: reduction or removal by the government of restrictions on a classified document or weapon
diminution: the act of decreasing or reducing something

Other hypernyms of declassification include decrease, reduction, step-down.

declassification (noun)	diminution (noun)
reduction or removal by the government of restrictions on a classified document or weapon	the act of decreasing or reducing something
	the statement of a theme in notes of lesser duration (usually half the length of the original)
	change toward something smaller or lower