# Fraud Proof

> Evidence that an optimistic rollup batch was invalid, used to roll back the bad batch.

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

## Definition

A fraud proof is the mechanism that keeps optimistic rollups honest. During the challenge window, anyone watching the rollup can submit a proof that a posted batch contains an invalid transaction.

The main chain re-runs the disputed step, and if the challenge is correct, the batch is reverted and the operator loses their bond. Fraud proofs only work if at least one honest party is watching, which is why optimistic rollups assume the existence of an active community of verifiers.

## Related terms

- [Optimistic Rollup](https://fudfomo.co/glossary/optimistic-rollup): A type of layer 2 that assumes batches are valid by default and lets anyone challenge them.
- [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.

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