Optimistic Rollup

An optimistic rollup is a Layer 2 scaling design that bundles many transactions off-chain and posts the results to the base blockchain, assuming they are valid by default. This optimistic assumption is what gives the design its name. Anyone can challenge a posted batch during a dispute window by submitting a fraud proof; if the challenge succeeds, the incorrect batch is reverted and the operator can be penalised. Because of that challenge period, withdrawals back to Layer 1 typically take about a week unless you use a third-party liquidity provider. Optimistic rollups are cheap and broadly compatible with existing Ethereum tooling, which is why many popular Layer 2 networks use them.