A Day 'Til Canceled (DTC) order is similar to a Day order, but instead of being canceled and then removed from the trading screen at the end of the day the order is "Deactivated" and set to a Saved/Paused state. This means that the order is canceled at the exchange, but remains "Saved" on your trading screen. After the end of the current Day session, the order status color will be gray for "Deactivated" and will allow the order to be re-transmitted by clicking the Transmit button the following day.
For more information, please visit the TWS Users' Guide.