Is an order to buy or sell on the close of a market. Typically, there is a brief period prior to the day's cessation of trading which is defined as the close. This period varies from market-to-market and exchange-to-exchange.
An order specification that requires the broker to get the best price available on the close of trading, usually during the last five minutes of trading.
an order to buy or sell at whatever price prevails at the end of the trading session