An order that is posted for execution only during the same trading day or session in which is submitted to an exchange/ trading floor. That is, this order is only good for execution or filling for a matter of hours during the respective day. Day orders get automatically cancelled at the end of a trading day (at the closing bell).

In that sense, day orders are similar to good until cancelled orders (GTC orders), with a slight difference: day orders remain good only for the same day of issue, then get cancelled at the end of that day, while GTC orders remain open and available for execution until an order to cancel them is submitted during the same day or following days.