A good-till-cancelled order (GTC) is an order that remains in force from one trading session to the next until it is either executed or withdrawn by the person who placed it. It contrasts with a day order, which lapses at the close if unfilled. The instruction is used mostly with a limit order or a stop order set at a price away from the current market, so that the trader need not re-enter it each day.
In practice the order does not last forever. Most brokers and exchanges cancel GTC orders automatically after a fixed period, commonly between 30 and 90 days. The risk of a long-lived order is that it is forgotten and then filled after news has changed the picture, for example a buy limit triggered by a sharp fall on bad results. Brokers may also adjust or cancel open orders after a dividend or a stock split.