What is error? What is error? An act, assertion, or belief that unintentionally deviates from what is correct, right, or true.The condition of having incorrect or false knowledge.The act or an instance of deviating from an accepted code of behavior.A mistake.Mathematics The difference between a computed or measured value and a true or theoretically correct value.Baseball A defensive fielding or throwing misplay by a player when a play normally should have resulted in an out or prevented an advance by a base runner.