Slippage

The difference between the expected price of a trade and the price at which it executes.

Slippage occurs when an order fills at a different price than intended, usually due to volatility or low liquidity between order entry and execution. It can be positive or negative. Market orders and large positions are most exposed; limit orders eliminate it at the cost of execution certainty.

His market order suffered slippage, filling a few cents worse than the price he saw on screen.