The difference between the expected price of a trade and the actual price at which it is executed. More common during high volatility and news events.
Slippage can be positive (better fill than expected) or negative (worse fill). In prop trading, slippage typically adds $1-5 per lot per trade in costs. During high-impact news events, slippage can be 5-20+ pips, which is why some firms restrict news trading. Account for slippage in your cost calculations — PropWise's Overtrading Detector includes slippage/spread costs in its analysis.