GTC

Good (Un)Til Cancelled. You have a broker who buys and sells stocks for you, and you get to tell them what stocks you want to buy and sell. A GTC is one type of instruction you can give your broker; it basically tells the broker to keep trying to make a trade until the trade goes through or until you call them up to cancel.

Example

You call your broker and tell them to buy 1,000 shares of stocks in a banana company because—you've decided to go vegan. If you've indicated that the order is GTC, the broker keeps trying to buy those 1,000 shares until they make the buy or until you call and tell them "Cancel that banana order. I've gone Paleo instead."