A cancel order is a message asking a broker or exchange to withdraw an order that is still waiting to be executed. It applies to resting orders, such as a limit order in the order book, and to the unfilled remainder of an order that has been partly executed. Quantity already traded cannot be cancelled. The venue confirms the cancellation, and until that confirmation arrives the order can still be filled, so a cancel request may arrive too late.
Cancelling is a normal part of trading. Market makers cancel and re-enter quotes continuously as prices move, often through a single replace order that changes price or size. Because very high cancellation rates load exchange systems, many venues monitor an order-to-trade ratio and charge or restrict members who exceed a limit. Entering orders with the intention of cancelling them before execution, in order to mislead other traders, is spoofing and is prohibited.