It stands for immediate-or-cancel order; an order to buy or sell a security subject to an instruction to execute immediately or otherwise the order gets cancelled. All or any portion of an IOC order that cannot be filled immediately gets cancelled, i.e., no more rests in the order book.

Once an investor/ trader places an IOC order to the market, the order has to be filled, either fully (and if not, it gets cancelled) or as fully as possible (and then, the unfilled portion gets cancelled). For example, if an investor sends an IOC order to buy 15,00 stocks of XYZ, if the executable number of stocks is only 1,000, then the IOC order gets filled only for 1,000 stocks, while the remaining portion, 500 stocks, is not filled, and hence it gets cancelled. In which case, the order is said to be partially filled, while part of it goes unexecuted.