Is a type of execution when a client is placing an order and specifies both volume and price; the order should be processed instantly.
If the price changes at that moment, a broker cannot change the execution price. But he can reject the execution and reply with a requote, which can be accepted or not. This execution type is usually used by market maker brokers.
Instant execution is often complemented by fixed spread, which is, as a rule, larger than real market spreads. The use of this execution type by market makers is caused by the fact that some trading platforms do not provide the possibility to work with the Depth of Market (DOM).