With regard to futures contracts as well as other financial instruments, slippage is the difference between where the computer signaled the entry and exit for a trade and where actual clients, with actual money, entered and exited the market using the computer’s signals.The left hand side of the image contains the market depth for the current BID prices and the right hand side of the image contains the market depth for the current ASK prices. Each side of the image contains three columns.
Slippage refers to the difference between the expected price of a trade and the price at which the trade is executed. Slippage can occur at any time but is most prevalent during periods of higher volatility when market orders are used. It can also occur when a large order is executed but there isn't enough volume at the chosen price to maintain the current bid/ask spread.
Slippage does not denote a negative or positive movement because any difference between the intended execution price and actual execution price qualifies as slippage. When an order is executed, the security is purchased or sold at the most favorable price offered by an exchange or other market maker. This can produce results that are more favorable, equal to or less favorable than the intended execution price. The final execution price vs. intended execution price can be categorized as positive slippage, no slippage and/or negative slippage.
Market prices can change quickly, allowing slippage to occur during the delay between a trade being ordered and when it is completed. The term is used in many market venues but definitions are identical. However, slippage tends to occur in different circumstances for each venue.