Slippage
The difference between the price expected when an order is placed and the price at which it actually fills.
Slippage comes from the quote moving between decision and execution, from market orders filling at the far side of a wide spread, and from orders larger than the displayed size that fill at several prices. It is usually larger in illiquid contracts and around fast moves.
Limit orders cap slippage at the limit price in exchange for the risk of not filling. Backtests that assume fills at the midpoint can overstate results when actual fills would pay part of the spread.
Illustrative example (hypothetical numbers, not a real trade): an option is quoted $1.95 bid and $2.05 ask with a $2.00 midpoint. A market order for 50 contracts fills 20 at $2.05 and 30 at $2.08 as the displayed size runs out. The average fill of $2.068 is $0.068 above the midpoint, about $340 across the order.