Optimistic Rollup

An optimistic rollup is a Layer-2 scaling solution that processes transactions off-chain and posts the results to the main blockchain, assuming all transactions are valid by default (hence "optimistic"). If someone suspects an invalid transaction, they can submit a fraud proof during a challenge window to dispute it. Optimistic rollups significantly increase throughput and reduce fees while inheriting the security of the underlying blockchain. Arbitrum and Optimism are well-known implementations.