An instruction by a customer to his/her broker to not carry out an order that he or she had placed earlier. A cancel order can only be placed in cases where the earlier order has not been executed. Further, if the broker has already submitted the order to a specialist or market maker at an exchange, then there might be a time lag (usually very small) in reaching the cancel order to the specialist or market maker, and the order might be executed before the customer's instruction to cancel is received by the specialist or market maker.