A type of attack on a decentralized network where a group gains control of the majority of nodes. This would allow them to defraud the blockchain by reversing transactions and double spending ether and other tokens.
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).
Most generally, this represents an EOA or contract that can receive (destination address) or send (source address) transactions on the blockchain. More specifically, it is the rightmost 160 bits of a Keccak hash of an ECDSA public key.
application binary interface (ABI)
The standard way to interact with contracts in the Ethereum ecosystem, both from outside the blockchain and for contract-to-contract interactions.
application programming interface
An Application Programming Interface (API) is a set of definitions for how to use a piece of software. An API sits between an application and a web server, and facilitates the transfer of data between them.
Application-specific integrated circuit. This usually refers to an integrated circuit, custom-built for cryptocurrency mining.
assert(false) compiles to
0xfe, an invalid opcode, which uses up all remaining gas and reverts all changes. When an
assert() statement fails, something very wrong and unexpected is happening, and you will need to fix your code. You should use
assert() to avoid conditions that should never, ever occur.
A claim made by an entity that something is true. In context of Ethereum, consensus validators must make a claim as to what they believe the state of the chain to be. At designated times, each validator is responsible for publishing different attestations that formally declare this validator's view of the chain, including the last finalized checkpoint and the current head of the chain.
Every block has a reserve price known as the 'base fee'. It is the minimum gas fee a user must pay to include a transaction in the next block.
The Beacon Chain was the blockchain that introduced proof-of-stake and validators to Ethereum. It ran alongside the proof-of-work Ethereum Mainnet from December 2020 until the two chains were merged in September 2022 to form the Ethereum of today.
A positional number representation where the most significant digit is first in memory. The opposite of little-endian, where the least significant digit is first.
A block is a bundled unit of information that include an ordered list of transactions and consensus-related information. Blocks are proposed by proof-of-stake validators, at which point they are shared across the entire peer-to-peer network, where they can easily be independently verified by all other nodes. Consensus rules govern what contents of a block are considered valid, and any invalid blocks are disregarded by the network. The ordering of these blocks and the transactions therein create a deterministic chain of events with the end representing the current state of the network.
An interface that allows a user to search for information from, and about, a blockchain. This includes retrieving individual transactions, activity associated with specific addresses and information about the network.
The block header is a collection of metadata about a block and a summary of the transactions included in the execution payload.
The process of transmitting a confirmed block to all other nodes in the network.
The specific validator chosen to create a block in a particular slot.
The amount of ether rewarded to the proposer of a new valid block.
The states that a block can exist in. The possible states include:
- proposed: the block was proposed by a validator
- scheduled: validators are currently submitting data
- missed/skipped: the proposer did not propose a block within the eligible time frame.
- orphaned: the block was reorg'd out by the fork choice algorithm
The time interval between blocks being added to the blockchain.
The process of checking that a new block contains valid transactions and signatures, builds on the heaviest historical chain, and follows all other consensus rules. Valid blocks are added to the end of the chain and propagated to others on the network. Invalid blocks are disregarded.
A sequence of blocks, each linking to its predecessor all the way to the genesis block by referencing the hash of the previous block. The integrity of the blockchain is crypto-economically secured using a proof-of-stake-based consensus mechanism.
The nodes which can be used to initiate the discovery process when running a node. The endpoints of these nodes are recorded in the Ethereum source code.
An abstract instruction set designed for efficient execution by a software interpreter or a virtual machine. Unlike human-readable source code, bytecode is expressed in numeric format.
The first of two hard forks for the Metropolis development stage. It included EIP-649 Metropolis Difficulty Bomb Delay and Block Reward Reduction, where the Ice Age was delayed by 1 year and the block reward was reduced from 5 to 3 ether.
Casper-FFG is a proof-of-stake consensus protocol used in conjunction with the LMD-GHOST fork choice algorithm to allow consensus clients to agree on the head of the Beacon Chain.
The Beacon Chain has a tempo divided into slots (12 seconds) and epochs (32 slots). The first slot in each epoch is a checkpoint. When a supermajority of validators attests to the link between two checkpoints, they can be justified and then when another checkpoint is justified on top, they can be finalized.
Converting code written in a high-level programming language (e.g., Solidity) into a lower-level language (e.g., EVM bytecode).
A group of at least 128 validators assigned to validate blocks in each slot. One of the validators in the committee is the aggregator, responsible for aggregating the signatures of all other validators in the committee that agree on an attestation. Not to be confused with sync committee.
A process is computationally infeasible if it would take an impracticably long time (eg. billions of years) to do it for anyone who might conceivably have an interest in carrying it out.
When a supermajority of nodes on the network all have the same blocks in their locally validated best blockchain. Not to be confused with consensus rules.
Consensus clients (such as Prysm, Teku, Nimbus, Lighthouse, Lodestar) run Ethereum's proof-of-stake consensus algorithm allowing the network to reach agreement about the head of the Beacon Chain. Consensus clients do not participate in validating/broadcasting transactions or executing state transitions. This is done by execution clients.
Ethereum's consensus layer is the network of consensus clients.
The block validation rules that full nodes follow to stay in consensus with other nodes. Not to be confused with consensus.
Considered for Inclusion (CFI)
A Core EIP which is not yet active on Mainnet, and client developers are generally positive towards the idea. Assuming it meets all the requirements for mainnet inclusion, it could potentially be included in a network upgrade (not necessarily the next one).
The second part of the Metropolis stage, originally planned for mid-2018. Expected to include a switch to a hybrid proof-of-work/proof-of-stake consensus algorithm, among other changes.
An account containing code that executes whenever it receives a transaction from another account (EOA or contract).
contract creation transaction
A special transaction that includes a contract's initiation code. The recipient is set to
null and the contract is deployed to an address generated from the user address and
nonce. that is used to register a contract and record it on the Ethereum blockchain.
The economics of cryptocurrencies.
Đ (D with stroke) is used in Old English, Middle English, Icelandic, and Faroese to stand for an uppercase letter “Eth”. It is used in words like ĐEV or Đapp (decentralized application), where the Đ is the Norse letter “eth”. The uppercase eth (Ð) is also used to symbolize the cryptocurrency Dogecoin. This is commonly seen in older Ethereum literature but is used less often today.
DAG stands for Directed Acyclic Graph. It is a data structure composed of nodes and links between them. Before The Merge, Ethereum used a DAG in its proof-of-work algorithm, Ethash, but is no longer used in proof-of-stake.
Decentralized application. At a minimum, it is a smart contract and a web user interface. More broadly, a dapp is a web application that is built on top of open, decentralized, peer-to-peer infrastructure services. In addition, many dapps include decentralized storage and/or a message protocol and platform.
The property of a state that any node connected to the network could download any specific part of the state that they wish to.
The concept of moving the control and execution of processes away from a central entity.
decentralized autonomous organization (DAO)
A company or other organization that operates without hierarchical management. DAO may also refer to a contract named "The DAO" launched on April 30, 2016, which was then hacked in June 2016; this ultimately motivated a hard fork (codenamed DAO) at block 1,192,000, which reversed the hacked DAO contract and caused Ethereum and Ethereum Classic to split into two competing systems.
decentralized exchange (DEX)
A type of dapp that lets you swap tokens with peers on the network. You need ether to use one (to pay transactions fees) but they are not subject to geographical restrictions like centralized exchanges – anyone can participate.
The gateway to staking on Ethereum. The deposit contract is a smart contract on Ethereum that accepts deposits of ETH and manages validator balances. A validator cannot be activated without depositing ETH into this contract. The contract requires ETH and input data. This input data includes the validator public key and withdrawal public key, signed by the validator private key. This data is needed for a validator to be identified and approved by the proof-of-stake network.
Short for "decentralized finance," a broad category of dapps aiming to provide financial services backed by the blockchain, without any intermediaries, so anyone with an internet connection can participate.
A network-wide setting in proof-of-work networks that controls how much average computation is required to find a valid nonce. The difficulty is represented by the number of leading zeroes that are required in the resulting block hash for it to be considered valid. This concept is deprecated in Ethereum since the transition to proof-of-stake.
Planned exponential increase in proof-of-work difficulty setting that was designed to motivate the transition to proof-of-stake, reducing the chances of a fork. The difficulty bomb was deprecated with the transition to proof-of-stake.
A short string of data a user produces for a document using a private key such that anyone with the corresponding public key, the signature, and the document can verify that (1) the document was "signed" by the owner of that particular private key, and (2) the document was not changed after it was signed.
The process by which an Ethereum node finds other nodes to connect to.
distributed hash table (DHT)
A data structure containing
(key, value) pairs used by Ethereum nodes to identify peers to connect to and determine which protocols to use to communicate.
A deliberate blockchain fork, where a user with a sufficiently large amount of mining power/stake sends a transaction moving some currency off-chain (e.g. exiting into fiat money or making an off-chain purchase) then reorganizing the blockchain to remove that transaction. A successful double spend leaves the attacker with both their on and off-chain assets.
elliptic curve digital signature algorithm (ECDSA)
A cryptographic algorithm used by Ethereum to ensure that funds can only be spent by their owners. It's the preferred method for creating public and private keys. Relevant for account address generation and transaction verification.
Encryption is the conversion of electronic data into a form unreadable by anyone except the owner of the correct decryption key.
In the context of cryptography, lack of predictability or level of randomness. When generating secret information, such as private keys, algorithms usually rely on a source of high entropy to ensure the output is unpredictable.
A period of 32 slots, each slot being 12 seconds, totalling 6.4 minutes. Validator committees are shuffled every epoch for security reasons. Each epoch has an opportunity for the chain to be finalized. Each validator is assigned new responsibilities at the start of each epoch.
A validator sending two messages that contradict each other. One simple example is a transaction sender sending two transactions with the same nonce. Another is a block proposer proposing two blocks at the same block height (or for the same slot).
'Eth1' is a term that referred to Mainnet Ethereum, the existing proof-of-work blockchain. This term has since been deprecated in favor of the 'execution layer'. Learn more about this name change.
'Eth2' is a term that referred to a set of Ethereum protocol upgrades, including Ethereum's transition to proof-of-stake. This term has since been deprecated in favor of the 'consensus layer'. Learn more about this name change.
Ethereum Improvement Proposal (EIP)
A design document providing information to the Ethereum community, describing a proposed new feature or its processes or environment (see ERC).
Ethereum Name Service (ENS)
The ENS registry is a single central contract that provides a mapping from domain names to owners and resolvers, as described in EIP 137.
Execution clients (formerly known as "Eth1 clients"), such as Besu, Erigon, Go-Ethereum (Geth), Nethermind, are tasked with processing and broadcasting transactions and managing Ethereum's state. They run the computations for each transaction using the Ethereum Virtual Machine to ensure that the rules of the protocol are followed.
Ethereum's execution layer is the network of execution clients.
externally owned account (EOA)
Externally owned accounts (EOAs) are accounts that are controlled by private keys, typically generated using a seed phrase. Unlike smart contracts, externally owned accounts are accounts without any code associated with them. Typically these accounts are managed with a wallet.
Ethereum Request for Comments (ERC)
A label given to some EIPs that attempt to define a specific standard of Ethereum usage.
A proof-of-work algorithm that was used on Ethereum before it transitioned to proof-of-stake.
The native cryptocurrency used by the Ethereum ecosystem, which covers gas costs when executing transactions. Also written as ETH or its symbol Ξ, the Greek uppercase Xi character.
Ethereum Virtual Machine (EVM)
A stack-based virtual machine that executes bytecode. In Ethereum, the execution model specifies how the system state is altered given a series of bytecode instructions and a small tuple of environmental data. This is specified through a formal model of a virtual state machine.
EVM assembly language
A human-readable form of EVM bytecode.
A default function called in the absence of data or a declared function name.
A service carried out via smart contract that dispenses funds in the form of free test ether that can be used on a testnet.
Finality is the guarantee that a set of transactions before a given time will not change and can't be reverted.
A denomination of ether. 1 finney = 1015 wei. 103 finney = 1 ether.
A change in protocol causing the creation of an alternative chain or a temporal divergence into two potential block paths.
The algorithm used to identify the head of the blockchain. On the execution layer the head of the chain is identified as the one with the greatest total difficulty behind it. This means the true head of the chain is the one that required the most work to mine it. On the consensus layer the algorithm observes the accumulated attestations from validators (LMD_GHOST).
A security model for certain layer 2 solutions where, to increase speed, transactions are rolled up into batches and submitted to Ethereum in a single transaction. They are assumed valid but can be challenged if fraud is suspected. A fraud proof will then run the transaction to see if fraud took place. This method increases the amount of transactions possible while maintaining security. Some rollups use validity proofs.
The initial test development stage of Ethereum, which lasted from July 2015 to March 2016.
A virtual fuel used in Ethereum to execute smart contracts. The EVM uses an accounting mechanism to measure the consumption of gas and limit the consumption of computing resources (see Turing complete).
The maximum amount of gas a transaction or block may consume.
Price in ether of one unit of gas specified in a transaction.
The first block in a blockchain, used to initialize a particular network and its cryptocurrency.
Go Ethereum. One of the most prominent implementations of the Ethereum protocol, written in Go.
Read more at geth.ethereum.org
Short for gigawei, a denomination of ether, commonly utilized to price gas. 1 gwei = 109 wei. 109 gwei = 1 ether.
A permanent divergence in the blockchain; also known as a hard-forking change. One commonly occurs when nonupgraded nodes can't validate blocks created by upgraded nodes that follow newer consensus rules. Not to be confused with a fork, soft fork, software fork, or Git fork.
A fixed-length fingerprint of variable-size input, produced by a hash function. (See keccak-256).
The number of hash calculations made per second by computers running mining software.
A wallet using the hierarchical deterministic (HD) key creation and transfer protocol.
HD wallet seed
A value used to generate the master private key and master chain code for an HD wallet. The wallet seed can be represented by mnemonic words, making it easier for humans to copy, back up, and restore private keys.
The second development stage of Ethereum, launched in March 2016 at block 1,150,000.
A network structure meant to optimize the querying of information from across the blockchain by providing an efficient path to its storage source.
Inter-exchange Client Address Protocol (ICAP)
An Ethereum address encoding that is partly compatible with the International Bank Account Number (IBAN) encoding, offering a versatile, checksummed, and interoperable encoding for Ethereum addresses. ICAP addresses use a new IBAN pseudo-country code- XE, standing for "eXtended Ethereum," as used in nonjurisdictional currencies (e.g., XBT, XRP, XCP).
A hard fork of Ethereum at block 200,000 to introduce an exponential difficulty increase (aka difficulty bomb), motivating a transition to proof-of-stake.
integrated development environment (IDE)
A user interface that typically combines a code editor, compiler, runtime, and debugger.
immutable deployed code problem
Once a contract's (or library's) code is deployed, it becomes immutable. Standard software development practices rely on being able to fix possible bugs and add new features, so this represents a challenge for smart contract development.
A transaction sent from a contract account to another contract account or an EOA (see message).
The minting of new ether to reward block proposal, attestation and whistle-blowing.
key derivation function (KDF)
Also known as a "password stretching algorithm," it is used by keystore formats to protect against brute-force, dictionary, and rainbow table attacks on passphrase encryption, by repeatedly hashing the passphrase.
Every account’s private key/address pair exists as a single keyfile in an Ethereum client. These are JSON text files which contains the encrypted private key of the account, which can only be decrypted with the password entered during account creation.
Cryptographic hash function used in Ethereum. Keccak-256 was standardized as SHA-3.
An area of development focused on layering improvements on top of the Ethereum protocol. These improvements are related to transaction speeds, cheaper transaction fees, and transaction privacy.
An open source on-disk key-value store, implemented as a lightweight, single-purpose library, with bindings to many platforms.
A special type of contract that has no payable functions, no fallback function, and no data storage. Therefore, it cannot receive or hold ether, or store data. A library serves as previously deployed code that other contracts can call for read-only computation.
An Ethereum client that does not store a local copy of the blockchain, or validate blocks and transactions. It offers the functions of a wallet and can create and broadcast transactions.
The fork-choice algorithm used by Ethereum's consensus clients to identify the head of the chain. LMD-GHOST is an acronym standing for "Latest Message Driven Greediest Heaviest Observed SubTree" which means that the head of the chain is the block with the greatest accumulation of attestations in its history.
Short for "main network," this is the main public Ethereum blockchain. Real ETH, real value, and real consequences. Also known as layer 1 when discussing layer 2 scaling solutions. (Also, see testnet).
Memory hard functions are processes that experience a drastic decrease in speed or feasibility when the amount of available memory even slightly decreases. An example is the Ethereum mining algorithm Ethash.
Merkle Patricia trie
A data structure used in Ethereum to efficiently store key-value pairs.
An internal transaction that is never serialized and only sent within the EVM.
The act of passing a message from one account to another. If the destination account is associated with EVM code, then the VM will be started with the state of that object and the message acted upon.
The third development stage of Ethereum, launched in October 2017.
The process of repeatedly hashing a block header while incrementing a nonce until the result contains an arbitrary number of leading binary zeros. This is the process by which new blocks are added to a proof-of-work blockchain. This was how Ethereum was secured before it moved to proof-of-stake.
A network node that finds valid proof-of-work for new blocks, by repeated pass hashing (see Ethash). Miners are no longer part of Ethereum - they were replaced by validators when Ethereum moved to proof-of-stake.
Minting is the process of creating new tokens and bringing them into circulation so that they can be used. It's a decentralized mechanism to create a new token without the involvement of the central authority.
Referring to the Ethereum network, a peer-to-peer network that propagates transactions and blocks to every Ethereum node (network participant).
The collective hashrate produced by an entire mining network. Mining on Ethereum was switched off when Ethereum moved to proof-of-stake.
non-fungible token (NFT)
Also known as a "deed," this is a token standard introduced by the ERC-721 proposal. NFTs can be tracked and traded, but each token is unique and distinct; they are not interchangeable like ETH and ERC-20 tokens. NFTs can represent ownership of digital or physical assets.
A software client that participates in the network.
In cryptography, a value that can only be used once. An account nonce is a transaction counter in each account, which is used to prevent replay attacks.
ommer (uncle) block
When a proof-of-work miner finds a valid block, another miner may have published a competing block which is added to the tip of the blockchain first. This valid, but stale, block can be included by newer blocks as ommers and receive a partial block reward. The term "ommer" is the preferred gender-neutral term for the sibling of a parent block, but this is also sometimes referred to as an "uncle". This was relevant for Ethereum when it was a proof-of-work network, but ommers are not a feature of proof-of-stake Ethereum because precisely one block proposer is selected in each slot.
A rollup of transactions that use fraud proofs to offer increased layer 2 transaction throughput while using the security provided by Mainnet (layer 1). Unlike Plasma, a similar layer 2 solution, Optimistic rollups can handle more complex transaction types – anything possible in the EVM. They do have latency issues compared to Zero-knowledge rollups because a transaction can be challenged via the fraud proof.
An oracle is a bridge between the blockchain and the real world. They act as on-chain APIs that can be queried for information and used in smart contracts.