Crypto glossary
Optimistic rollup
An optimistic rollup is a layer 2 network that posts transactions to Ethereum and assumes they are valid unless someone proves fraud within a set window.
An optimistic rollup executes transactions off the main chain, bundles them and publishes the data plus a claimed new state to Ethereum. It does not prove up front that each batch is correct. Instead there is a challenge period, typically about seven days, during which anyone watching can submit a fraud proof showing a batch was wrong, and the invalid state is thrown out.
Arbitrum, Optimism and Base are the best-known optimistic rollups. Because they run the EVM, Ethereum applications can be deployed on them with few changes. Users who withdraw through the official bridge must wait for the challenge window to close, which is why third-party fast bridges exist.
Security depends on at least one honest party checking the chain and on fraud proofs actually working; several rollups launched with these systems disabled or limited to approved parties. Many still rely on a single sequencer that orders transactions and could censor users or go offline.
Example
Withdrawing 1 ETH from Arbitrum to Ethereum through the native bridge takes about a week; a liquidity bridge can deliver it in minutes in exchange for a small fee.
