Reductio ad absurdum
Reductio ad absurdum is a mode of argumentation that seeks to establish a contention by deriving an absurdity from its denial, thus arguing that a thesis must be accepted because its rejection would be untenable. It is a style of reasoning that has been employed throughout the history of mathematics and philosophy from classical antiquity onwards.
Or the standard dictionary definition
Main Entry:reÂ·ducÂ·tio ad abÂ·surÂ·dum
Pronunciation:\ri-ËˆdÉ™k-tÄ“-ËŒÅ-ËŒad-É™b-ËˆsÉ™r-dÉ™m, -ËˆdÉ™k-sÄ“-Å-, -shÄ“-, -ËˆzÉ™r-\
Etymology:Late Latin, literally, reduction to the absurd
1 : disproof of a proposition by showing an absurdity to which it leads when carried to its logical conclusion
2 : the carrying of something to an absurd extreme