In
mathematics, a
proof in which you assume temporarily that the
conclusion is not
true, and then deduce a
contradiction.
For example:
Given: n is an integer and n^2 is even.
Prove: n is even.
Proof
Asume temporarily that n is not even. Then n is odd, and n^2 = n * n = odd * odd = odd. But this contradicts the given information that n^2 is even. Therfore the temporary assumption that n is not even must be false. It follows that n is even.