A rollup is a Layer 2 scaling design that executes transactions off-chain (or on a separate chain) and posts the data or a proof of correctness back to a Layer 1 for security.
Two flavors:
- Optimistic rollups (Optimism, Arbitrum, Base) post the transactions and assume they are valid. Anyone can submit a fraud proof during a challenge window (typically 7 days) to revert invalid state.
- ZK rollups (zkSync, Starknet, Polygon zkEVM, Linea, Scroll) post a cryptographic proof that the off-chain execution was correct. No challenge window needed once the proof is verified on L1.
Rollups inherit L1 data availability and (with caveats) censorship resistance. They do not necessarily inherit L1 decentralization, most rollups today have a centralized sequencer, a multi-sig admin key, and an upgrade path that can change the contract logic. Read the L2Beat risk profile before treating an L2 as having mainnet-equivalent security.