Slippage
The difference between the price a trade was expected to execute at and the price it actually executed at.
Slippage occurs when a trade fills at a different price than requested — usually during fast-moving markets, low liquidity, or around major news events. It can work against a trader (negative slippage) or, less often, in their favor (positive slippage).
An EA sends a buy order at 2,428.50 on XAUUSD, but by the time the broker's server processes it, the price has moved to 2,428.80 — a slippage of 0.30, or 30 points, against the trader.
Why it matters
Backtests that ignore slippage tend to overstate real-world performance, especially for high-frequency or scalping strategies where trade costs are a larger share of each trade's expected profit. Realistic backtesting always models a slippage assumption alongside spread and commission.