Layer 2
A network on top of the blockchain that executes transactions cheaply and only summarises the result downwards.
The base layer is expensive and slow by design — every node must verify everything. A layer 2 instead runs transactions off to the side and periodically writes down a summary, together with a proof or an opportunity to dispute. Security is thereby inherited from the underlying chain rather than rebuilt.
Two families dominate on Ethereum. Optimistic rollups assume the summary is correct and allow a week-long challenge period, which makes withdrawals slow. ZK-rollups attach a cryptographic proof and need no waiting period, but are computationally heavier. Bitcoin's equivalent, the Lightning Network, is built on payment channels rather than rollups.