It means an order to buy or sell stock that is good until you execute or cancel it. Brokerages usually set a limit of 30-60 days, at which the G.T.C. order expires if not restated. (In contrast to a day order.)
GOOD 'TIL CANCELLED ORDER (G.T.C.)
Meaning of GOOD 'TIL CANCELLED ORDER (G.T.C.) in English
Campbell R. Harvey. Hypertextual finance English glossary. Английский словарь гипертекстовых финансов. 2012