L2 (Layer 2)

A blockchain that derives its security from an underlying L1 it commits transactions to.

A Layer 2 is a blockchain that runs on top of an L1 (usually Ethereum), processing transactions more cheaply but periodically committing state to the L1 for security. Optimistic rollups (Optimism, Arbitrum) assume validity and challenge fraud; zk-rollups (zkSync, Starknet, Linea, Scroll) prove validity cryptographically.

Not every L2 is genuinely a rollup. Some chains marketed as L2s commit only minimal data to Ethereum (a "validium") or have centralized sequencers that could censor or freeze withdrawals. The L2 Beat dashboard tracks the actual security properties of each project.