An optimistic rollup is a Layer-2 scaling solution designed to enhance the throughput and efficiency of blockchains, particularly Ethereum. It allows transactions to be processed off the Layer-1 network, with the assumption that off-chain transactions are valid by default, thus considered ‘optimistic’.
Optimistic rollup operators bundle multiple off-chain transactions together in large batches before submitting to the Layer-1 chain. These rollups use a scheme called ‘fraud-proving’ to challenge invalid transactions, where, after a rollup batch is posted, there is a time window (also known as the ‘challenge period’) for anyone to submit a fraud proof to challenge the results of the rollup transactions.
Additionally, optimistic rollups also use compression techniques to lower the amount of data; as a result, users enjoy lower transaction fees. Notable examples of optimistic rollups include Arbitrum, Optimism, and Blast.
