split down vs diminution

split down and diminution both are nouns.

Word	Noun	Adjective	Verb	Adverb
split down	Yes	No	No	No
diminution	Yes	No	No	No

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

split down: a decrease in the number of outstanding shares of a corporation without changing the shareholders' equity
diminution: the act of decreasing or reducing something

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

split down (noun)	diminution (noun)
a decrease in the number of outstanding shares of a corporation without changing the shareholders' equity	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