Immediate Or Cancel Order - IOC

An order requiring that all or part of the order be executed immediately after it has been brought to the market. Any portions not executed immediately are automatically cancelled.

Investopedia Says:
This is used for large orders where filling quickly can be difficult.

Related Links:
Taking control of your portfolio means knowing when to use particular orders and if they pose added costs. The Basics Of Order Entry

Order requiring that all or part of the order be executed as soon as the broker enters a bid or offer; the portion not executed is automatically canceled. Such stipulations usually accompany large orders.