On a highly volatile market, an order might be executed at a price that is different than expected. Let's see an example.
If bid-ask spread on BTC-EUR pair is €3460 by €3469, and you place a market order to buy 1 BTC, you may expect it to fill at €3469. In the fraction of a second, it takes for your order to reach the exchange something may change, or your quotes could be slightly delayed. The price you actually get maybe €3462. The €2 difference between your expected price of €3460 and the €3462 price you actually end up buying at is called slippage.
Slippage refers to the difference between the expected price of a trade and the price at which the trade is actually executed. Slippage often occurs during periods of higher volatility when market orders are used. A market order assures you get into the trade, but there is a possibility you will end up with slippage and a worse price than expected.
Slippage does not directly refer to a negative or positive movement, as any change between the expected and actual prices can qualify. When orders are executed, the corresponding securities are purchased or sold at the most favorable price available. This can cause an order to produce results that are more favorable, equal to or less favorable than original expectations with the results being referred to as positive slippage, no slippage and negative slippage, respectively.
Want to know more about Slippage? Check Investopedia article