Front-Running
Seeing a pending transaction and placing your own ahead of it to profit from the price move it will cause.
On a public blockchain, transactions wait in a mempool where anyone can read them before they are included in a block, and whoever orders the block decides the sequence. Searchers monitor pending transactions and submit their own with a higher priority fee, or bid directly to block builders, so that theirs executes first. The term is borrowed from equities, where front-running means a broker trading ahead of a client's order and is misconduct precisely because the order was confidential; on-chain, the pending transaction is public data and the extraction is done by unrelated parties, so it is a property of the design rather than a breach of a duty. That distinction does not make it costless to users, and it does not excuse the separate case where a centralized venue trades ahead of its own customers' orders, which is misconduct in the traditional sense. Mitigations include private transaction relays, encrypted mempools, batch auctions that clear many orders at one price, and tight slippage limits.
In practice
Priority fees exist partly because block space is auctioned, and the same auction that orders ordinary transactions is what lets a searcher place a trade immediately ahead of a pending swap.
The common misunderstanding
That on-chain front-running is the same offense as a broker trading ahead of a client, when the pending transaction is public information rather than confidential client data and no fiduciary relationship exists.