Use 'rebut' when you need to firmly deny what someone says, especially when you have proof. Politicians often rebut accusations in debates.
Pro Tip 2/3
Strong Defense
'Rebut' suggests a strong response or comeback to challenge a statement or belief. The scientist rebutted the theory with new evidence.
Pro Tip 3/3
Not Merely Denial
'Rebut' is stronger than just saying 'no' as it usually involves explaining why the other point is wrong. He didn't just say no; he rebutted every point with facts.