apagoge

from The Century Dictionary.

• noun In logic: Abduction (which see). The demonstration of a proposition by the refutation of its opposite (from Aristotle's ἡ εἰς το\ ἀδύνατον ἀπαγωγή, reduction to the impossible): commonly called indirect proof.
• noun In mathematics, a progress or passage from one proposition to another, when the first, having been demonstrated, is employed in proving the next.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Logic) An indirect argument which proves a thing by showing the impossibility or absurdity of the contrary.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun logic An indirect argument which proves a thing by showing the impossibility or absurdity of the contrary.