that refutes, contradicts, or otherwise disagrees with a something previously said.
A poor example, thought up in a matter of seconds:
John: LOLOL WHALES LAY EGGS!!11!!1one!
Martin: I disagree, whales do not lay eggs.
John: btw, was taht a rebuttal?
Martin: Yes. Yes it was.
A poor example:
John: HELLO R U THAR
Martin: Yes, I am here.
John: u just responded, rigt??/?
Martin: Yes. Yes I did.