Optimistic Rollup
A rollup that assumes transactions are valid unless challenged within a dispute window — the model behind Arbitrum and Base.
An optimistic rollup posts transaction data to the base chain and assumes it’s valid by default, relying on a challenge period (typically ~7 days) during which anyone can submit a fraud proof to contest invalid transactions. If unchallenged, the batch finalizes. Arbitrum, Optimism, and Base use this design.
The trade-off is the withdrawal delay: moving funds back to Layer 1 waits out the challenge window (unless you use a liquidity provider to front the funds for a fee). Optimistic rollups were simpler to build than zk-rollups and captured early market share, but zk designs are gradually catching up. The security model rests on at least one honest party watching and able to submit fraud proofs.