Arbitrum is an Ethereum Layer 2 scaling platform built on optimistic rollup technology. It processes transactions off-chain while inheriting security from Ethereum's base layer, enabling significantly higher throughput and lower transaction costs than Ethereum mainnet. Arbitrum One is the flagship production chain, implemented as a trustless, permissionless optimistic rollup. In this architecture, transactions are executed off-chain and batched together, with compressed transaction data and state commitments posted back to Ethereum for settlement. The protocol assumes transactions are valid unless challenged. If a dispute arises, fraud proofs and interactive challenge games are used to verify correctness, with Ethereum serving as the final arbitration layer. This design allows Arbitrum One to process transactions faster and cheaper than Ethereum while maintaining Ethereum-grade security guarantees.