MEV (Maximal extractable value)
In short MEV is the profit that block producers and bots capture by reordering, inserting, or censoring transactions inside a block, often at traders' expense.
MEV stands for maximal extractable value, originally miner extractable value. It is the extra profit someone can make by choosing which transactions go into a block and in what order. Because the order of trades changes the price each trade gets, controlling that order is worth money.
The most common forms are arbitrage, liquidations, and sandwich attacks. In a sandwich, a bot spots your pending swap in the mempool, buys just before you to push the price up, lets your trade fill at the worse price, then sells right after. Your slippage tolerance sets how much the bot can take. On Ethereum, searchers bundle these trades and pay builders and validators to include them; Solana has its own versions built around validator clients and bundle auctions.
It matters most when trading small tokens with thin liquidity, where one buy moves the price a lot. A loose slippage setting, like 20% or more, invites a bot to take nearly all of it. Not all MEV hurts users: arbitrage keeps prices aligned across pools, and liquidations keep lending markets solvent.
To limit exposure, keep slippage as low as the trade allows and split large orders. Many wallets and DEX aggregators offer private or protected transaction routing that keeps your swap out of the public mempool. If a trade fills far worse than quoted, check the block explorer: a buy and a sell from the same address right before and after yours is the classic sign of a sandwich.