Stop Loss
A resting order that closes a losing position automatically at a price you set in advance.
A stop-loss sits in the market below your entry on a long trade, or above it on a short. If price reaches that level, the order triggers and closes the position, capping the loss without requiring you to be at the screen.
A standard stop becomes a market order once triggered, so the fill is not guaranteed at your exact level - in a gap or a news spike it can be materially worse. A guaranteed stop-loss, offered by brokers such as CMC and Plus500 for a premium, fills at your level regardless.
Placement should be driven by market structure, not by the amount you feel like losing. A stop just beyond a recent swing high or low is a level where your trade idea is genuinely wrong; a stop placed at an arbitrary round number simply invites a random exit before the move you predicted.