MEV
The extra value a block producer can capture by choosing which transactions go into a block and in what order.
Common sources are arbitrage between venues after a large trade, liquidating undercollateralized loans first, and sandwiching a pending swap by trading immediately before and after it so the victim receives a worse price. The term began as miner extractable value and was renamed maximal extractable value once proof of stake and specialized builders took over block construction. Dashboards report MEV revenue, and the equity-style word is misleading in a specific way: this is value transferred from other users through control of ordering, not income earned by selling a service, and it does not correspond to profits, earnings, or anything a company would report. Mitigations under active development include private order flow, encrypted mempools, batch auctions that clear at a uniform price, and application designs that reduce the value of ordering.
In practice
A sandwich attack places one trade ahead of a pending swap and one behind it, so the original trader executes at a worse price and the extractor keeps the difference.
The common misunderstanding
MEV is not a flaw in one application; it arises wherever someone controls transaction ordering, which includes every chain with discretionary block building.