Glossary
Fraud Proof
A challenge submitted to an optimistic rollup's L1 contract that re-executes a disputed state transition to prove it was invalid, reverting the bad batch and slashing the proposer.
A fraud proof is how an optimistic rollup stays honest. Because the L1 contract accepts posted state as valid by default, security rests on any watcher's ability to dispute a bad batch during the challenge window. The proof re-executes the contested state transition on L1 — or interactively narrows it down to a single disputed step — and if the result differs from what the sequencer claimed, the batch is reverted and the proposer's bond is slashed.
This is the trust assumption that makes optimistic rollups work: you don't need everyone to verify every block, only that at least one honest party is watching and willing to submit a proof. Schemes split into single-round (non-interactive) and multi-round interactive designs like the one Arbitrum uses.