Definition
The difference between an expected or reference price and the actual execution price. It can be favorable or unfavorable.
Hypothetical example
A buy expected at 100 that fills at 101 has an adverse difference of 1, or about 1%.
Keep in mind
Slippage is separate from commissions. Omitting it can make a backtest look better than actual trading.
Official reference
www.investor.gov ↗Examples are illustrative assumptions, not investment recommendations.