Full Idea
A tableau proof is a proof by reduction ad absurdum. One begins with an assumption, and one develops the consequences of that assumption, seeking to derive an impossible consequence.
Gist of Idea
Tableau proofs use reduction - seeking an impossible consequence from an assumption
Source
David Bostock (Intermediate Logic [1997], 4.1)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.141