maximal extractable value

Maximal extractable value, or MEV, is the extra profit that whoever builds a block can capture purely by choosing which transactions go in and in what order, beyond the normal block reward and fees. Because a miner or validator decides the exact sequence of pending transactions, they hold a quiet but powerful lever: the order of trades and operations can itself be worth money, and that value can be extracted by arranging the block cleverly.

It arises because pending transactions sit in public view in the mempool before they are confirmed, and on a blockchain order is everything: which trade executes first can change prices and outcomes. A classic example is a front-run, where someone sees a large pending swap that will move a price, inserts their own trade just ahead of it, and profits from the move the victim's trade was about to cause. Related tactics include back-running and "sandwiching" a victim's trade between two of one's own.

Imagine a ticket clerk who can see every order in the queue and is free to rearrange them: they could slip their friend's purchase ahead of yours to grab the last good seat before the price jumps. MEV is the blockchain version of that ordering power, and it is an active research frontier: because it can quietly tax ordinary users and even threaten a chain's stability, much current work aims to make transaction ordering fairer, more private, or more transparent rather than a hidden source of profit.