Slippage
Slippage is the difference between the price you expected for an order and the price it actually filled at.
It happens when the price moves between your order and its execution, or when your order is larger than what sits at the best price in the order book and fills at several worse ones. A stop-loss can slip badly in a fast market.
Limit orders avoid slippage by fixing the worst price you accept, at the risk of not filling. On decentralised exchanges a slippage tolerance setting does the same job: the trade fails if the price moves further than you allow.