Slippage
Quick definition
The difference between the price you expected and the price your order actually fills at.
Slippage happens when the market moves — or liquidity runs thin — between placing your order and execution. Buy a thin altcoin with a market order and you might pay noticeably more than quoted.
It's worst in low-liquidity markets and during volatile moments. Limit orders cap the price you'll accept; on DEXs you can set a slippage tolerance.