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A comprehensive blockchain glossary covering 50 protocol-level terms including: Abstract Account, Archive Node, BFT consensus, Blob (EIP-4844), Bytecode, Canonical Chain, Circuit Breaker, Client Diversity, Cold Start, Commitment Scheme, Consensus Client, Execution Client, EIP, Finality, Fork Choice Rule, Gas Refund, Genesis Block, Hard Fork, Hot Path, Instruction Set, MEV, and many more. Essential reference for blockchain developers and technical readers.
Abstract Account
An Ethereum account type being formalized through EIP-3074 and EIP-7702, allowing EOAs to temporarily execute smart contract logic. Distinct from ERC-4337 smart accounts.
Archive Node
An Ethereum node storing complete historical state (every state at every block height). Required for `eth_getBalance` at a historical block. ~2TB+ storage. Used by Alchemy, Infura, Etherscan for historical queries.
BFT (Byzantine Fault Tolerant)
A consensus algorithm that remains safe even if some participants behave adversarially (not just crash). Requires 2/3+ honest participants. Used by Tendermint/CometBFT (Cosmos) and Hyperledger Fabric 3.0's BFT orderer.
Blob (EIP-4844)
A new transaction data type introduced in Ethereum's Dencun upgrade (March 2024). Blobs store L2 batch data with separate pricing from calldata. Each blob: ~125KB. Deleted after 18 days (not permanently stored by nodes).
Bytecode
The compiled representation of Solidity code that runs in the EVM. Bytecode is what actually gets deployed on-chain. `solc` compiles Solidity to EVM bytecode. Maximum contract size: 24,576 bytes.
Canonical Chain
The chain that all honest nodes consider the main chain. In Proof of Work: longest chain. In Proof of Stake: LMD-GHOST determines the canonical chain.
Circuit Breaker
An emergency mechanism that pauses a protocol when anomalous conditions are detected. Typically implemented as: if protocol TVL drops 30% in one hour OR oracle price changes 20% in one block, pause all operations.
Client Diversity
Having multiple independent implementations of an Ethereum node (Geth, Erigon, Nethermind, Besu for execution layer; Prysm, Lighthouse, Teku, Nimbus for consensus). Diversity prevents a single client bug from halting the entire network.
Cold Start
An account or storage slot accessed for the first time in a transaction. Cold SLOAD: 2,100 gas. Cold CALL: 2,600 gas. Subsequent accesses in same transaction are 'warm' (100 gas, 100 gas).
Commitment Scheme
A cryptographic primitive allowing a party to commit to a value without revealing it, then later reveal it with proof. Used in: ZK circuits (commit to inputs before proof), commit-reveal patterns, Chainlink VRF.
Consensus Client
The Ethereum node component responsible for Proof of Stake consensus (Prysm, Lighthouse, Teku, Nimbus). Communicates with the execution client via the Engine API.
EIP (Ethereum Improvement Proposal)
The formal specification process for proposed Ethereum changes. ERCs (token standards like ERC-20, ERC-721) are a subset of EIPs focused on application-layer standards.
Execution Client
The Ethereum node component responsible for executing transactions, maintaining state, and running the EVM (Geth, Erigon, Nethermind, Besu). Pairs with a consensus client.
Finality (Probabilistic)
Bitcoin's security model — transactions become progressively harder to reverse as more blocks are added. After 6 confirmations: extremely unlikely to be reversed. No guarantee of absolute irreversibility.
Fork Choice Rule
The algorithm validators use to select the canonical chain when multiple competing forks exist. Ethereum: LMD-GHOST (Latest Message Driven Greediest Heaviest Observed SubTree).
Gas Refund
ETH returned to the transaction sender when storage is cleared (setting a value from non-zero to zero). Currently: 4,800 gas refund per SSTORE to zero (reduced from 15,000 in EIP-3529).
Genesis Block
The first block in a blockchain. Block #0. Cannot be modified. Contains initial state (for testnets: pre-funded accounts; for mainnet: the Ethereum Foundation initial distribution).
Hard Fork
A backward-incompatible protocol upgrade. Nodes that don't upgrade cannot validate blocks from the new fork. Ethereum's major upgrades (The Merge, Shapella, Dencun) were all hard forks.
Instruction Set
The set of opcodes available in a virtual machine. EVM has ~150 opcodes (ADD, SSTORE, CALL, etc.). The EVM instruction set is stable — new EIPs add opcodes carefully to avoid breaking changes.