What is Immediate or cancel?

An immediate or cancel order, also known as an "accept order", is a finance term used in investment banking or securities transactions that refers "an order to buy or sell a stock that must be executed immediately". In case the entire order is not available at that moment for purchase a partial fulfillment is possible, but any portion of an IOC order that cannot be filled immediately is cancelled, eliminating the need for manual cancellation. This "partial fulfillment" aspect is what differentiates IOC orders from all or none and fill or kill orders, but the terms might be used interchangeably in some markets.