Stop-loss
Also called: stop loss, stop losses, stop-losses, stop-loss order
An exit set in advance: an order that triggers a sale once the price falls to the level where you decided your idea was wrong.
A stop-loss order has a trigger price. Nothing happens until the market touches it. Then the order goes to the exchange, either as a limit order at a price you chose (SL) or as a market order (SL-M). Many brokers do not offer the market version for options.
The useful part is the decision, not the order type. Choosing the exit before you buy makes you say what would prove you wrong, and it lets you size the position so that being wrong costs a small, fixed share of your money.
Example
You buy a share at 500 and decide that below 460 your reason for owning it no longer holds. You place a sell stop-loss with a trigger of 460 and a limit of 455. If the price drops to 460, the order becomes a sell limit at 455.
Watch out
A stop-loss does not guarantee your price. If a share opens far below your trigger after bad news, a limit order may not fill at all, and a market order fills wherever the market happens to be.
Learn it properly in
Chapter 2.1: Position sizing and risk: the maths that keeps you in the game