A Fill or Kill (FOK) order is a type of trade order that must be executed immediately in its entirety or cancelled completely. Unlike partial fills, an FOK order ensures that the trader either gets the full quantity at the specified price or nothing at all.
Example
“The hedge fund manager placed a FOK order to buy 50,000 shares at exactly $120.50 — because only 40,000 were available at that price, the entire order was killed and no shares were purchased.”