turn down vs rebuff

turn down is not a noun while rebuff is a noun.

Word	Noun	Adjective	Verb	Adverb
turn down	No	No	Yes	No
rebuff	Yes	No	Yes	No

As verbs, rebuff is a hyponym of turn down; that is, rebuff is a word with a more specific, narrower meaning than turn down:

Other hyponyms of turn down include repel, snub.

turn down (noun)	rebuff (noun)
	a deliberate discourteous act (usually as an expression of anger or disapproval)
	an instance of driving away or warding off

turn down (verb)	rebuff (verb)
make lower or quieter	reject outright and bluntly
take a downward direction	force or drive back
reject with contempt
not accept as true
refuse entrance or membership