Optimistic Rollup

A layer-2 scaling solution where transactions are assumed valid unless someone proves otherwise via a fault proof. Arbitrum and Optimism (OP Mainnet) are the dominant production examples; Base, Blast, Mantle, and many others are also optimistic rollups. The 7-day withdrawal delay is the trade-off — fast and EVM-equivalent in execution, but exits to L1 wait for the challenge window. Cheaper than ZK rollups in proving cost, with weaker cryptographic security guarantees.