# Optimistic Rollup

> A type of layer 2 that assumes batches are valid by default and lets anyone challenge them.

Canonical URL: https://fudfomo.co/glossary/optimistic-rollup
Source: What The Block! Dictionary v1.0 (last updated 2026-04-25), browsable at https://wtb.fudfomo.co.

## Definition

An optimistic rollup posts batches of transactions back to its layer 1 with the assumption that they are correct. There is a challenge window, often a week, during which anyone can submit a fraud proof if they spot a bad batch.

If a fraud proof succeeds, the bad batch is rolled back and the operator is penalised. The trade-off is that withdrawing funds back to the main chain takes the length of that challenge window.

## Related terms

- [Rollup](https://fudfomo.co/glossary/rollup): A type of layer 2 that bundles many transactions together and writes a summary back to the main chain.
- [Layer 2](https://fudfomo.co/glossary/layer-2): A faster, cheaper network built on top of a layer 1 like Ethereum. Borrows the main chain's security.
- [Fraud Proof](https://fudfomo.co/glossary/fraud-proof): Evidence that an optimistic rollup batch was invalid, used to roll back the bad batch.
- [Ethereum](https://fudfomo.co/glossary/ethereum): A blockchain that lets developers build apps and issue tokens. Often called a programmable money platform.

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