An order that is not completely filled, i.e., it is partially executed. This means that only a part of the order has been executed, while the remainder remains in the exchange system as an open order (waiting to be filled.)
Generally, the fraction filled depends on the difference between buy order volume and sell order volume. For example, if buy order volume is double that of sell order, the system will fill only half the total volume of all buy orders. The percentage of filled orders, or the so-called fill rate, depends for a specific order on a set of exchange rules, as applicable at the time of placing an order at a given market. Additionally, there is a set of matching rules that governs a situation where sell orders are larger than buy orders (in terms of volume).
