It stands for intermarket sweep order; a type of sweep orders, specifically a limit order, that automatically fills in a designated market center, irrespective of quotations in other market centers- even if better quotations are made available there. Being multi-exchange orders, intermarket sweep orders (ISOs) sweep different markets centers and pick up the necessary number of securities, either from one center, if possible, or otherwise from as many centers as need for a complete fill. The order sweeps the market place, usually using an algorithm, identifying as many securities as possible at the best prices available.

An investor submitting an ISO must meet order protection rules set by an exchange by simultaneously posting orders to the markets with better prices.