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An order from a broker to buy or sell a security or derivative, usually in large quantities, in which investors have a short period of time to partially or completely fulfill the order before it is cancelled. An IOC may be considered the opposite of a good 'til cancelled (GTC) order; it differs from a fill-or-kill (FOK) order because an IOC may only be partially filled, while an FOK must be entirely filled or the whole offer is void. An IOC is considered a type of day order, but has a much shorter time frame. See also: All-or-none offering.
A customer order that not only specifies a price but also requires the broker to fill immediately as much of the order as possible and cancel any part remaining. This type of order differs from a fill-or-kill order, which disallows a partial fill.