What is MEV?
Also called: maximal extractable value, miner extractable value
MEV is the profit available from choosing the order, inclusion, or exclusion of transactions in a block. It covers benign arbitrage as well as extraction that comes directly out of user trades.
In practice
Because pending transactions are public, searchers can front-run or sandwich them. Mitigations include private transaction relays, batch auctions, and encrypted mempools. MEV research is a specialised, well-paid niche that sits between protocol engineering and market microstructure.