Glossary
Arbitrum
An optimistic rollup developed by Offchain Labs — Ethereum's largest L2 by TVL, running a fraud-proof system on a custom EVM-compatible runtime (Nitro).
Arbitrum One launched in 2021 and quickly became the most-used L2 on Ethereum. It is an optimistic rollup: transactions are batched and posted to L1, with a seven-day challenge window where anyone can submit a fraud proof if the posted state is wrong.
The current runtime, Nitro, executes a slightly-modified EVM and geth, so most contracts deploy unchanged. Arbitrum Orbit lets teams spin up their own L2/L3 chains backed by Arbitrum's tech.