Optimistic Rollup

An optimistic rollup is a type of rollup that "assume[s] offchain transactions are valid and don't publish proofs of validity for batches of transactions," relying instead on a fraud-proof challenge system to catch errors. After a batch of transactions is posted to layer 1, a challenge window—commonly around seven days—opens during which anyone running a full node can dispute the batch with a fraud proof; if unchallenged, the batch is treated as final. That window is why withdrawing funds from an optimistic rollup back to Ethereum normally takes about a week, unlike a ZK rollup, which finalizes as soon as its validity proof is verified. Arbitrum, Optimism, and Base are the largest optimistic rollups by usage. For tokenized domains held on an optimistic rollup, that challenge window is a practical detail worth knowing: a transfer can be usable immediately on the rollup but not fully final against layer 1 until the window closes.