It stands for intermarket sweep order; an order that sweeps across the exchange (exchanges) where it is placed, picking up as many securities as possible at the best prices (highest for a sell order and lowest for a buy order) available using a dedicated algorithm. It is generally a large quantity limit order that is placed across multiple exchanges simultaneously. An example is a market designation on an IOC limit order that allows the order to sweep the order book without considering any external quotes.

It provides a digital exception to the “order protection rule“, stipulating that orders (buy orders) have to be filled at the lowest price possible.