Slippage is the difference between the expected price of your trade, and the actual price at which it is executed. After you’re quoted a price on a trade and you confirm it, that trade enters a limbo state among other Ethereum transactions that are awaiting to be settled upon the mining of a new block on Ethereum’s network.

If there are any other transactions in limbo that are also targeting the same liquidity pool, this could change the price at which your transaction is settled at, depending on the order in which these transactions were sequenced. This difference in price is known as slippage. Usually, users aren’t aware of each other's concurrent trading, which results in an equal likelihood of either positive or negative slippage. 

What is an MEV attack?

While the average user isn’t aware of the trades others are making, there are others with malicious intentions that are actively watching the sequencing of transactions and participating using bots in re-ordering of the transaction prior to block mining in order to seize opportunities. This is known as an MEV attack, which is essentially when participants in the ecosystem use privileged information to trade against the end user, impacting that end-users final price.