A type of option order which requires that the order be executed completely or not at all. A fill-or-kill order is similar to an all-or-none (AON) order. The difference is that if the order cannot be completely executed (i.e., filled in its entirety) as soon as it is announced in the trading crowd, it is to be 'killed' (i.e., cancelled) immediately. Unlike an AON order, a FOK order cannot be used as part of a GTC order.