MEV (maximal extractable value)

MEV, or maximal extractable value, is the profit that block producers and specialised traders can make by choosing which transactions to include in a block and in what order. It was originally called "miner extractable value".

Common forms include arbitrage between decentralised exchanges, liquidations in lending protocols and "sandwich attacks". In a sandwich attack, a bot sees your large swap waiting in the public mempool, buys the token just before you to push the price up, lets your trade execute at the worse price, then sells immediately after. You suffer extra slippage; the bot pockets the difference.

On Ethereum, most blocks are now built by specialised builders through a system called MEV-Boost. Users can reduce MEV harm by setting tight slippage limits, using DEX aggregators or wallets with MEV protection, and sending transactions through private relays that keep them out of the public mempool until they are included in a block.