An immediate or cancel order (IOC order) must be cancelled if not immediately executed. Unlike a fill or kill order, it may be part filled.
Market orders usually expire immediately anyway, so the explicit use of the term immediate or cancel implies that it is being applied to a limit order.
An IOC order gives immediate certainty on whether an order executes or not. This may be advantageous, for example, when an investor wishes to use the money elsewhere if a security is not available at the desired price.