Reductio ad absurdum (Latin for “reduction to the absurd”), also known as an apagogical argument, reductio ad impossibile, or proof by contradiction, is a type of logical argument where one assumes a claim for the sake of argument and derives an absurd or ridiculous outcome, and then concludes that the original claim must have been wrong as it led to an absurd result.
It makes use of the law of non-contradiction — a statement cannot be both true and false. In some cases it may also make use of the law of excluded middle — a statement must be either true or false. The phrase is traceable back to the Greek ἡ εἰς ἄτοπον ἀπαγωγή (hē eis átopon apagōgḗ), meaning “reduction to the absurd”, often used by Aristotle. (via Wikipedia)
An example off the top of my head:
John: I believe God determines what is right and wrong, good and evil.
Jack: In that case, you must believe that all atheists are immoral reprobates since they don’t believe in God!
(In this case, though, I would argue that the reductio doesn’t follow.)