Definition - Noun 1: an unintentional error esp. in legal procedure or form that does not indicate bad faith and that commonly warrants excuse or relief by the court <the court's power to revise a judgment because of fraud, ~, or irregularity> <a clerical ~> 2: an erroneous belief: as a: a state of mind that is not in accordance with the facts existing at the time a contract is made and that may be a ground for the rescission or reformation of the contract b: a misconception at the time of an offense alleged by a defendant