Stop loss
A stop loss is a postponed order that closes a losing position at a price you set in advance. It is usually a stop-market order, below the price for a long and above it for a short.
When price trades at the stop, the order becomes a market order and closes the trade, often a little beyond the stop because of slippage. Plan for that: the stop price is your intended exit, not a guaranteed one.
Learn more: types of orders and where to put your stop loss.