A trading market order for buying or selling that must be filled in its entirety or not at all.
A fill or kill order (FOK) is a market order to buy or sell all the specified units of an asset or none at all. The order is similar to the All or Nothing (AON) order that also revolves around the goal to fill up a buy or sell order or remain inactive. The shared goal of both of these orders is to maximize profit at once without risking the odds of getting partial orders, some of which may be accompanied by different prices.
A FOK is usually a good strategy. However, to use it effectively, a trader has to be confident about price and volume movements of the asset(s) in question.