Slippage

Slippage in a backtest is modeled as a small cost that always moves the fill against you — worse than expected on a buy, worse than expected on a sell — applied to every simulated entry, stop, and target fill. Real fills can also slip for reasons a backtest doesn't model, such as low liquidity or fast-moving prices, so backtest results still tend to overstate what live trading would produce even with slippage modeled in.