A fill-or-kill order (FOK) is an order that must be executed immediately and for its whole quantity. If the market cannot fill the full amount at once at the stated price or better, the entire order is cancelled and nothing trades. It is a time-in-force condition, normally attached to a limit order, and it never rests in the order book.
The condition combines two requirements that other order types apply separately. An immediate-or-cancel order also demands instant execution but accepts a partial fill and cancels only the remainder. An all-or-none order also demands a complete fill but may wait for it. Traders use fill-or-kill when a partial position would be unwanted, for example when one order is a leg of a multi-part trade that only makes sense if every leg is done in full, or when a large buyer wants to take visible liquidity without revealing unfilled demand.