rejection vs rebuff

rejection and rebuff both are nouns.

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

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

Other hyponyms of rejection include renunciation, repudiation, repulse, snub, short shrift, summary treatment.

rejection (noun)	rebuff (noun)
the act of rejecting something	a deliberate discourteous act (usually as an expression of anger or disapproval)
the speech act of rejecting	an instance of driving away or warding off
(medicine) an immunological response that refuses to accept substances or organisms that are recognized as foreign
the state of being rejected

rejection (verb)	rebuff (verb)
	reject outright and bluntly
	force or drive back