Appendix E: Glossary

This glossary contains terms used throughout this book and in the broader EVM ecosystem.

A

Account An object containing an address, balance, nonce, and optional storage and code. An account can be a contract account or an externally owned account (EOA).

Agentic Coding AI-assisted software development using autonomous agents that can generate, review, test, and deploy code. Particularly valuable for resource-constrained development teams. See Chapter 18.

Address A 20-byte (160-bit) identifier for an account. Derived from the rightmost 160 bits of the Keccak-256 hash of an ECDSA public key.

Assert In Solidity, assert(false) compiles to 0xfe, an invalid opcode that consumes all remaining gas. Use assert() for conditions that should never occur—indicates a bug if triggered.

B

Big-endian A byte ordering where the most significant byte is stored first. The EVM uses big-endian for most operations.

Block A collection of transactions, a block header, and references to ommer blocks. Blocks are added by miners (PoW) or validators (PoS).

Blockchain A sequence of blocks, each cryptographically linked to its predecessor, forming an immutable ledger.

Bytecode Low-level instructions executed by the EVM. Compiled from high-level languages like Solidity or Vyper.

C

Consensus Agreement among nodes on the current state of the blockchain.

Consensus Client In post-Merge Ethereum (PoS), the client responsible for the Proof of Stake consensus mechanism. Works alongside an execution client. Examples: Lighthouse, Prysm, Teku. Not required on PoW chains like Ethereum Classic.

Consensus Rules The validation rules that full nodes follow to stay in consensus with other nodes.

CLAUDE.md A configuration file placed in a project root that provides context, principles, and constraints to Claude Code. Used to encode network-specific development guidelines for agentic coding workflows.

Contract Account An account containing code that executes when it receives transactions. Has no private key—controlled only by its code.

Core Geth The primary execution client for Ethereum Classic, maintained as a fork of go-ethereum (Geth). Supports ETC mainnet (chainId 61) and Mordor testnet (chainId 63).

D

DAO Decentralized Autonomous Organization. Also refers to "The DAO" contract hacked in June 2016, which led to the ETH/ETC split.

DApp Decentralized Application. A web application built on decentralized infrastructure, typically combining smart contracts with a frontend.

Difficulty In Proof of Work, a network-wide setting controlling how much computation is required to mine a block.

Digital Signature Cryptographic proof that a message was created by the holder of a specific private key.

E

ECDSA Elliptic Curve Digital Signature Algorithm. The cryptographic algorithm used to sign transactions and prove ownership of accounts.

EIP Ethereum Improvement Proposal. A design document proposing new features or changes. See eips.ethereum.org.

ENS Ethereum Name Service. Maps human-readable names (like vitalik.eth) to addresses.

EOA Externally Owned Account. An account controlled by a private key, used by humans to interact with the blockchain.

ERC Ethereum Request for Comments. A category of EIPs defining application-level standards (e.g., ERC-20 for tokens).

Ethash The original Proof of Work algorithm used by Ethereum before The Merge and still used (as ETCHash) by Ethereum Classic.

ETCHash The PoW mining algorithm used by Ethereum Classic since the Thanos upgrade (November 2020). A modification of Ethash that supports 4GB GPUs.

Ether The native cryptocurrency of Ethereum and Ethereum Classic networks. Symbol: ETH (Ethereum) or ETC (Ethereum Classic).

Ethereum Classic (ETC) The original Ethereum blockchain that continued without the DAO hard fork. Maintains Proof of Work consensus. ChainId 61 (mainnet), 63 (Mordor testnet).

EVM Ethereum Virtual Machine. The stack-based virtual machine that executes bytecode. Shared by Ethereum, Ethereum Classic, and other EVM-compatible chains.

Execution Client Client software responsible for executing transactions and managing state (the EVM). On PoW chains like ETC, only an execution client is needed. Examples: Geth, Core Geth, Besu, Erigon.

F

Fallback Function A contract function called when no other function matches the call data, or when the contract receives plain ether.

Faucet A service that dispenses free test ether for use on testnets.

Finney A denomination of ether. 1 finney = 10^15 wei = 0.001 ether.

Fork A change in protocol rules, or a divergence creating two competing chains. Can be "soft" (backward compatible) or "hard" (not backward compatible).

Foundry A Rust-based Ethereum development toolkit. Includes Forge (testing), Cast (CLI), Anvil (local testnet), and Chisel (REPL).

G

Gas A unit measuring computational effort in the EVM. Each operation costs a specific amount of gas.

Gas Limit The maximum gas a transaction can consume (set by sender) or a block can contain (set by network).

Gas Price The amount of ether (in wei) the sender pays per unit of gas.

Genesis Block The first block (block 0) of a blockchain.

Geth Go Ethereum. The most widely used Ethereum client, written in Go.

H

Hardhat A popular Ethereum development framework for JavaScript/TypeScript, featuring Hardhat Network for local testing.

Hard Fork A protocol change that is not backward compatible. Nodes must upgrade or they will follow a different chain.

Human-in-the-Loop A development pattern where AI agents perform tasks but require human approval at critical checkpoints. Essential for production smart contract deployment where errors can be irreversible.

Hash A fixed-size output produced by a cryptographic hash function from variable-size input. Used extensively for data integrity and identification.

HD Wallet Hierarchical Deterministic wallet. Uses a seed phrase to derive unlimited keys following BIP-32/39/44 standards.

I

ICAP Inter-exchange Client Address Protocol. An address encoding compatible with IBAN, using the XE prefix.

Immutability The principle that deployed smart contract code cannot be changed. A core property of blockchain systems.

Internal Transaction A message sent from one contract to another during execution. Not recorded as a separate transaction on-chain.

IPFS InterPlanetary File System. A decentralized storage protocol often used with DApps for storing content.

K

Keccak-256 The cryptographic hash function used by Ethereum. Related to but not identical to SHA-3.

Keystore File A JSON file containing an encrypted private key, protected by a password.

L

Library A contract deployed once and called by other contracts using DELEGATECALL. Cannot hold state or receive ether.

Light Client A client that doesn't store the full blockchain, instead requesting data from full nodes as needed.

M

Mainnet The primary, production blockchain network. Ethereum mainnet (chainId 1) and ETC mainnet (chainId 61).

Merkle Patricia Tree The data structure used by Ethereum to store state, transactions, and receipts efficiently.

Message An internal transaction between contracts, never serialized to the blockchain.

Miner A node that finds valid Proof of Work solutions for new blocks. Used on PoW chains like Ethereum Classic.

MCP (Model Context Protocol) A protocol developed by Anthropic for connecting AI agents to external tools and data sources. Enables Claude Code to interact with blockchain nodes, databases, and other services.

Mordor The primary testnet for Ethereum Classic. Uses PoW consensus (ETCHash) and chainId 63.

N

NFT Non-Fungible Token. A unique token following the ERC-721 or ERC-1155 standard.

Node A software client that participates in the blockchain network.

Nonce A counter used to prevent replay attacks. Account nonce: counts transactions sent. Block nonce (PoW): the value miners vary to find valid hashes.

O

Ommer A valid block that wasn't included in the main chain. Previously called "uncle." Ommer miners receive partial rewards.

Opcode A single instruction in EVM bytecode. See Appendix C for the complete reference.

P

Private Key A 256-bit secret number that controls an account. Must be kept secure—anyone with the key controls the account.

Proof of Stake (PoS) A consensus mechanism where validators stake cryptocurrency to participate in block production. Used by Ethereum since The Merge (September 2022).

Proof of Work (PoW) A consensus mechanism requiring computational work to produce blocks. Used by Ethereum Classic and Bitcoin.

Public Key A point on the secp256k1 elliptic curve derived from a private key. Used to verify signatures and derive addresses.

R

Receipt Data returned after transaction execution, including status, gas used, and logs.

Reentrancy A vulnerability where a contract can be called back before its first execution completes, potentially draining funds.

Revert An operation that undoes all state changes in the current call and returns remaining gas.

RLP Recursive Length Prefix. Ethereum's serialization format for encoding arbitrary data structures.

S

Selfdestruct An opcode that destroys a contract and sends its balance to a specified address. Deprecated in future upgrades.

Sepolia The primary testnet for Ethereum development. Uses PoS consensus and chainId 11155111.

Smart Contract A program stored on the blockchain that executes automatically when triggered by transactions.

Solidity The most popular language for writing smart contracts. Syntax similar to JavaScript/C++.

SSTORE The opcode for writing to contract storage. One of the most expensive operations (20,000 gas for new values).

Szabo A denomination of ether. 1 szabo = 10^12 wei = 0.000001 ether.

T

Testnet A blockchain network for testing, using valueless test ether. Active testnets: Sepolia (ETH), Mordor (ETC).

The Merge The September 2022 upgrade that transitioned Ethereum from PoW to PoS. Ethereum Classic did not participate.

Transaction A signed message sent from an EOA, targeting an address with optional value and data.

Turing Complete A system capable of computing anything computable given enough resources. The EVM is Turing complete (bounded by gas).

V

Validator In PoS networks, a participant who stakes cryptocurrency to propose and attest to blocks. Ethereum validators stake 32 ETH.

Vyper A Python-like smart contract language designed for security and simplicity.

W

Wallet Software managing private keys and providing an interface to interact with the blockchain.

Web3 The vision of a decentralized web built on blockchain technology.

Wei The smallest denomination of ether. 1 ether = 10^18 wei.

Z

Zero Address The address 0x0000000000000000000000000000000000000000. Used as the destination for contract creation transactions.

---

Denomination Reference

Unit	Wei Value	Ether Value
Wei	1	10^-18
Kwei (babbage)	10^3	10^-15
Mwei (lovelace)	10^6	10^-12
Gwei (shannon)	10^9	10^-9
Szabo	10^12	10^-6
Finney	10^15	10^-3
Ether	10^18	1

Chain IDs

Network	Chain ID
Ethereum Mainnet	1
Ethereum Classic	61
Sepolia (ETH testnet)	11155111
Mordor (ETC testnet)	63