Trading on the stock exchange is characterized by rapid and immediate fluctuations in the price
of securities. If an order is placed and execution takes place at a price different than the price just displayed on the order form, this is referred to as slippage. Slippage therefore refers to an immediate "slip“ of the price during order execution. It’s the difference between the expected price that the trader expects to pay for a trade and the actual price at which the trade is made.
Slippage only occurs with market orders and stop market orders. By contrast, when placing a limit order, you define a fixed price limit. Therefore, It isn’t possible for execution to take place if the price is less favorable than the price limit.