diminution vs step-down

diminution and step-down both are nouns.

Word	Noun	Adjective	Verb	Adverb
diminution	Yes	No	No	No
step-down	Yes	No	No	No

As nouns, diminution and step-down are synonyms defined as:

Other synonyms of diminution include decrease, reduction.

diminution (noun)	step-down (noun)
the act of decreasing or reducing something	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