Slippage

Slippage is the difference between the price you expect when you place a trade and the price you actually get. It happens when the market moves between your click and execution, or when your order is too large for the liquidity at the best price and has to fill at progressively worse prices.

On centralised exchanges, you can avoid slippage by using limit orders, which only execute at your price or better, at the cost of possibly not filling. On decentralised exchanges, you set a slippage tolerance, for example 0.5%, and the transaction fails rather than executing at a worse rate.

Setting a very high slippage tolerance on a DEX is risky. Bots watching the mempool can "sandwich" your trade, buying before you and selling after you to capture the difference.