Broadly speaking, slippage refers to a failure to meet expectation with regard to the execution of an order.

Slippage reflects the extent to which an orders fill price differs negatively from the price level at which it was entered. For example if a sell stop loss order was placed at 1.2762 in the eur/usd and the order was filled at 1.2755, one would have experienced 7 pips of slippage on the order.

Slippage also describes the difference between estimated transaction costs for a trade and the amount paid due to market conditions, poor execution etc.