A Fill or Kill (FOK) order is an instruction to execute a trade immediately and completely at the specified price, or cancel the entire order if it cannot be filled in full. This order type is used when traders need certainty of full execution and will not accept partial fills.
Example
“The institutional trader submitted a Fill or Kill order for 10,000 shares at $50.00, but the exchange could only match 8,000 shares at that price, so the entire order was automatically cancelled.”