L2 (Layer 2)
A Layer 2 is a chain that inherits its security from a Layer 1, usually by posting transaction data and proofs there. Optimistic rollups (Arbitrum, Optimism) and ZK rollups (zkSync, Linea) are the two dominant models. L2s exist mostly because Ethereum L1 cannot scale execution by itself without sacrificing decentralization.
See also: L1 (Layer 1), Protocol, Rollup.
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