An order you put with your broker that turns into a market order if the price hits the stop point. It’s typically called a stop (or stop-loss order) because most traders use it to make sure they sell an open position before it gets away from them. It typically will stop a loss from getting too big. However, since it turns into a market order when the stop price is hit, you are not guaranteed that you’ll get that price. It might be much worse. Most electronic brokerage systems will allow you to put a stop order into their computer. The computer then sends it out as a market order when that price is hit. Thus, it does not go into the market where everyone might see it and look for it.