Good 'till canceled order (GTC)

Basically, this is what it says it is - an order that is good until it is canceled by the trader. This type of order is usually used in conjunction with a limit order - e.g. sell whenever the stock hits X unless I cancel the order before then. Typically though these orders will only stay open a maximum of 90 days because brokers don't want to accumulate millions of these orders indefinitely.

One other thing to note is that whenever a stock that has a GTC order on it goes ex-dividend the limit price is automatically adjusted downward by the amount of the dividend.