tax credit vs step-down

tax credit and step-down both are nouns.

Word	Noun	Adjective	Verb	Adverb
tax credit	Yes	No	No	No
step-down	Yes	No	No	No

As nouns, step-down is a hypernym of tax credit; that is, step-down is a word with a broader meaning than tax credit:

tax credit: a direct reduction in tax liability (not dependent on the taxpayer's tax bracket)
step-down: the act of decreasing or reducing something

Other hypernyms of tax credit include decrease, diminution, reduction.

tax credit (noun)	step-down (noun)
a direct reduction in tax liability (not dependent on the taxpayer's tax bracket)	the act of decreasing or reducing something