An indirect debate which proves something by showing the impossibility or absurdity for the contrary.
An indirect argument which shows something by showing the impossibility or absurdity associated with the contrary.
In reasoning: Abduction (which see). The demonstration of a proposition by the refutation of the opposite (from Aristotle's ἡ εἰς το\ ἀδύνατον ἀπαγωγή, reduction into the impossible): generally called indirect evidence.
In mathematics, a progress or passageway in one proposition to another, whenever very first, having already been demonstrated, is required in showing the second.