ZK-rollup
A layer 2 that bundles thousands of transactions and submits a cryptographic proof that all of them were valid.
The proof can be verified on the main chain far more cheaply than re-running the transactions, which is the entire scaling gain. Because validity is mathematically proven, no challenge period is needed — withdrawals can happen immediately, unlike optimistic rollups where you wait a week.
The name is partly misleading. Zero-knowledge proofs are really about proving something without revealing the underlying data, but here the technique is used mainly for its compressing property; the privacy is a side effect that is not always exploited. Generating the proofs is computationally heavy, and running arbitrary smart contracts in a zk environment has been the hard part.