slippage vs diminution

slippage and diminution both are nouns.

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

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

Other hypernyms of slippage include decline.

slippage (noun)	diminution (noun)
failing to hold or slipping out of place	the act of decreasing or reducing something
a decrease of transmitted power in a mechanical system caused by slipping	the statement of a theme in notes of lesser duration (usually half the length of the original)
decline from a standard level of performance or achievement	change toward something smaller or lower