Stop-loss
A price level where you plan to exit a trade to limit a loss. An actual stop-loss order may not execute at exactly that price.
A stop-loss is a price you choose before entering a trade where you will get out if the market goes against you. It turns "I hope this works" into a plan with a limit on how much you can lose.
A good stop goes where your idea is proven wrong, such as just below a support zone you are relying on, not at a random round number. Moving a stop further away because a trade is losing is one of the most common ways small losses become large ones.
In fast markets a stop can fill at a worse price than planned, so size the trade as if that might happen. In TRADECRAFT, every simulated trade asks for a stop before it can be placed.
Example: you plan to buy at $50 because $48 has been support. You set a stop-loss at $47.50, just under that zone, so the trade closes if support fails.
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Educational definition. Not financial advice; no real-money execution.