It stands for immediate or cancel (IOC) intermarket sweep order (ISO); a market or limit order that is immediately executed, in whole or part, while the unexecuted amount is canceled (not routed to other exchanges for any fills), allowing the order to sweep the book without any consideration to any external quotes.

In other words, this order will be immediately and automatically executed against the displayed bid (offer) up to its full size in accordance with the rule defined, and to the extent set by the exchange, and will then sweep the exchange’s book and the amount so unfilled will be immediately and automatically cancelled.

This order is also known as an ISO designated IOC.