MEV
Profit extracted by reordering, inserting or censoring transactions in a block.
Whoever builds a block chooses the order of transactions inside it, and that ordering has monetary value. Searchers scan the mempool for opportunities such as arbitrage and sandwich attacks, then bid for favourable placement.
MEV is an unavoidable consequence of public mempools. Most of the mitigation work involves private order flow, encrypted mempools and fair-ordering research.