defend vs rebuff

defend is not a noun while rebuff is a noun.

Word	Noun	Adjective	Verb	Adverb
defend	No	No	Yes	No
rebuff	Yes	No	Yes	No

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

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

defend (verb)	rebuff (verb)
argue or speak in defense of	reject outright and bluntly
state or assert	force or drive back
be on the defensive; act against an attack
protect against a challenge or attack
protect or fight for as a champion
fight against or resist strongly
be the defense counsel for someone in a trial