From Traderpedia

Unfavorable price movements in price between the time the order is placed and when it is filled.

Slippage is the extent to which a fill deviates (negatively) from the level at which the order was entered. For example, in the case of a stop sell order triggered at 9438 which got filled at 9435, the slippage would be 3 ticks (or the equivalent cash amount). Slippage can occur in a fast market or as a result of a lack of liquidity.