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Complete Definition
Simple Definition
A comprehensive glossary of Layer 2 and scaling terminology for rollup developers and blockchain infrastructure engineers.
Key Definition
A comprehensive glossary of 30 Layer 2 and scaling terms including: Anytrust, Batch, Blob (EIP-4844), Calldata, Challenge Period, Data Availability (DA), Data Availability Layer, Decentralized Sequencer, Fault Proof, Finality (L2), Force Inclusion, Fraud Proof, L1, L2, Modular Blockchain, Native Bridge, Optimistic Rollup, Prover, Rollup, Sequencer, Settlement Layer, Shared Sequencer, Sovereign Rollup, State Diff, Superchain, Type 1/2/3/4 ZK-EVM, Validity Proof, Validium, Verifier, Withdrawal Delay, and ZK-Rollup.
Anytrust
Arbitrum's data availability mode where a committee attests to data availability rather than posting all data to Ethereum L1, trading some decentralization for lower costs.
Batch
A collection of L2 transactions compressed and posted together to L1 as a single data submission, amortizing the L1 posting cost across many transactions.
Blob (EIP-4844)
A data structure introduced for L2 batch posting, providing dramatically cheaper data availability than calldata after the Dencun upgrade.
Challenge Period
The time window (typically 7 days for optimistic rollups) during which a posted L2 state can be disputed via fraud proof before becoming final.
Data Availability (DA)
The guarantee that transaction data is published and retrievable, enabling anyone to reconstruct and verify L2 state independently.
Data Availability Layer
A specialized blockchain (Celestia, EigenDA) dedicated to providing cheap, scalable data availability for rollups, separate from execution.
Decentralized Sequencer
A goal/roadmap item for most major L2s — distributing transaction ordering authority across multiple parties rather than a single centralized operator.
Fraud Proof
A cryptographic proof submitted to L1 demonstrating that a specific L2 state transition was invalid, used in optimistic rollups to enable dispute resolution.
L1 (Layer 1)
The base blockchain — Ethereum mainnet, Bitcoin, Solana — providing the foundational security and settlement layer.
L2 (Layer 2)
A scaling solution built on top of an L1, processing transactions with greater throughput while inheriting L1 security guarantees.
Modular Blockchain
Architecture separating consensus, execution, settlement, and data availability into specialized layers, as opposed to monolithic chains that handle all functions in one layer.
Native Bridge
The official, protocol-level bridge connecting an L2 to its parent L1, typically the most trust-minimized (but slowest) way to move assets between layers.
Optimistic Rollup
A scaling solution that assumes transaction batches are valid by default, allowing a challenge period for fraud proofs rather than requiring upfront validity proofs.
Rollup
A general term for scaling solutions that execute transactions off-chain and post compressed data plus proof of validity (optimistic or ZK) to L1.
Sequencer
The entity responsible for ordering and batching transactions on an L2 before they're posted to L1, currently centralized for most major rollups.
Superchain
Optimism's vision/architecture for multiple OP Stack-based L2s sharing common infrastructure, security properties, and potential for native interoperability.
Validity Proof
A cryptographic proof (used by ZK-rollups) that mathematically demonstrates a batch of transactions was executed correctly, verified on L1 without needing a challenge period.
Validium
A scaling solution using validity proofs (like ZK-rollups) but storing data off-chain rather than posting to L1, trading some data availability guarantees for lower costs.
ZK-Rollup
A scaling solution using zero-knowledge proofs to validate transaction batches, achieving faster finality than optimistic rollups by proving correctness upfront rather than allowing a dispute period.
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