It stands for passive liquidity order; a non-displayed order that allows market participants to provide liquidity to the market without publicly revealing their trading plans or intentions (passive liquidity). It is an order to buy or sell a stated amount of a security at a specified, undisplayed/ non-displayed price. Passive liquidity orders are executed in the working order process after all other working orders except undisplayed discretionary order interest. Passive liquidity orders with a price better (on buy and sell sides) than (superior to) that of directed fills will have price priority and execution will take place with priority over inferior priced directed fills in the directed order process (DOP).

A passive liquidity order must be designated as an inside limit order (ILO). A passive liquidity order designated immediate or cancel (IOC) is automatically rejected.