Circular reasoning is providing evidence for the validity of an assertion, which assumes the validity of the assertion.
General forms include "A is true because A is true" or "A is true because B is true, and B is true because A is true".
Often used as a mechanism to prevent an assertion from being challenged or questioned, or to "win" a debate by sending it round and round in circles.
Examples of circular reasoning:
"I'm right because I'm right."
"There isn't a problem with the rule, because if everyone obeyed it there wouldn't be a problem."
"Piracy is wrong because it's against the law, and it's against the law because it's wrong."
"X is stupid because he's an idiot."