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:re·duc·tio ad ab·sur·dum
Pronunciation:\ri-ˈdək-tē-ˌō-ˌad-əb-ˈsər-dəm, -ˈdək-sē-ō-, -shē-, -ˈzər-\
Function:noun
Etymology:Late Latin, literally, reduction to the absurd
Date:1741
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