Definition:Circular Proof

Definition
A circular proof is a proof $A$ in terms of another proof $B$ such that the truth of $B$ depends (either directly or indirectly) on the truth of $A$.

A circular proof is valid, as every statement implies itself.

The problem of a circular proof is that if the truth of a conclusion is dubious, premisses that are equally dubious provide no reason to support the conclusion.

Also see

 * Valid Argument
 * Circular Definition