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What is a rollup?
A rollup is a layer 2 network that executes transactions away from a base chain such as Ethereum, then posts the transaction data back to it in batches. Because the data lands on the base chain, a rollup draws its security from that chain rather than from its own validators.
What it means for you
Sharing one base-chain posting among many users is what makes rollup fees lower. Withdrawing to the base chain is not always instant: on an optimistic rollup the funds wait through a challenge period of roughly seven days, unless a third party pays them out early for a fee.
How it works
A rollup has two parts: an operator that orders and executes transactions off Ethereum, and contracts on Ethereum that receive its batches. The batch data is posted to layer 1, either as calldata or as blobs that are pruned after about 18 days, so the data is on Ethereum if a dispute needs it. Optimistic rollups secure the result with fraud proofs and a challenge window; ZK-rollups with validity proofs checked by a contract. Posting the data to Ethereum is what separates a rollup from a sidechain.
Sources: ethereum.org: Scaling, ethereum.org: Optimistic rollups, ethereum.org: Sidechains · checked 4 October 2026
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