This glossary explains the terms you meet on ChainMonitor and in the learn articles. Each entry is short. Where a longer explanation exists, the term links to it. If you are new, start with what is a blockchain, and for common questions see the blockchain FAQ.
A
Account model
A way of tracking balances where each address has a balance that goes up and down, like a bank account. Ethereum, its layer 2 networks, Polygon and Solana use it. Bitcoin uses UTXOs instead.
Address
A string of characters that identifies where funds can be sent, derived from a public key. An EVM address starts with 0x. See wallets and keys.
Approval
Also called an allowance. A signed permission that lets a smart contract spend a set amount of one of your tokens. Review and revoke old approvals regularly.
B
Base fee
The part of an Ethereum-style fee that is set by the network from how full recent blocks were, and is burnt. It rises when blocks are more than half full and falls when they are less. Introduced by EIP-1559. See gas and fees.
Batch
A group of layer 2 transactions compressed together and posted to the layer 1 in one go. See rollups.
Blob
A temporary chunk of data attached to an Ethereum transaction under EIP-4844. Rollups use blobs to post their data cheaply. Blobs have their own fee market.
Block
A bundle of transactions added to a blockchain together, linked to the previous block by its hash. See what is a blockchain.
Block explorer
A website that lets you look up blocks, transactions and addresses on a chain. ChainMonitor includes one for each of its seven chains. See read a transaction.
Block height
The position of a block in the chain, counted from the first block at height 0.
Bridge
Software that moves value between two blockchains, usually by locking or burning tokens on one chain and minting them on the other. See bridges and their risks.
C
Calldata
The input data field of an Ethereum transaction. Rollups used it to post data to Ethereum before blobs existed. It is permanent and relatively costly.
Canonical bridge
The official bridge of a rollup, built into its design and run by contracts on Ethereum.
Challenge window
The period, about 7 days on the large optimistic rollups, during which anyone can dispute a withdrawal or result with a fraud proof. See optimistic vs zk rollups.
Compute unit
A measure of computation on Solana. Each transaction uses a number of compute units, and the priority fee is priced per unit. See Solana priority fees.
Confirmation
A new block added after the one that includes your transaction. More confirmations make a change less likely. See finality and confirmations.
Consensus
The process by which the nodes of a network agree on which blocks are valid and in what order. Examples are proof of work and proof of stake.
Contract
Short for smart contract. A program stored on a blockchain that runs when called by a transaction.
Custodial wallet
A wallet where a company holds the private keys for you, as at an exchange. See wallets and keys.
D
Decentralization
How widely control of a network is spread among many independent participants rather than a single party. It is a matter of degree, not a yes or no.
DeFi
Short for decentralized finance. Financial apps such as swaps and lending that run as smart contracts.
Difficulty
On Bitcoin, a number that sets how hard the mining puzzle is. It adjusts every 2016 blocks to keep blocks at about 10 minutes.
E
EIP
Ethereum Improvement Proposal, a numbered document describing a change to Ethereum. Proposals are listed at eips.ethereum.org.
EIP-1559
The Ethereum fee design that splits a fee into a base fee, which is burnt, and a priority fee paid to the validator. See gas and fees.
EIP-4844
An Ethereum upgrade that introduced blobs, which made data posting cheaper for rollups.
Epoch
A fixed group of slots. On Ethereum an epoch is 32 slots, which is 6.4 minutes. Solana also uses epochs, which are much longer.
ERC-20
The standard for fungible tokens on Ethereum and chains that work like it. USDC on Base is an ERC-20 token. See stablecoins.
EVM
Ethereum Virtual Machine, the system that runs smart contracts on Ethereum. Arbitrum, Base, Optimism and Polygon are EVM-compatible, so they share address formats and tools.
F
Finality
The point at which a transaction can no longer be reversed in practice. On Ethereum it takes about two epochs. See finality and confirmations.
Fraud proof
Evidence submitted to Ethereum that a rollup posted an invalid result. Used by optimistic rollups.
G
Gas
The unit that measures how much work a transaction needs on an EVM chain. You pay for gas in the chain's coin. See gas and fees.
Gas limit
The maximum amount of gas you allow a transaction to use. On Ethereum it is also the cap on how much gas a whole block can hold.
Gas price
The price you pay for each unit of gas, usually quoted in gwei. Total fee equals gas used times gas price.
Gwei
A unit of ether used for gas prices. One gwei is one billionth of an ETH, which is 1,000,000,000 wei.
H
Halving
An event on Bitcoin, about every 210,000 blocks, when the block reward for miners is cut in half.
Hardware wallet
A small device that stores private keys and signs transactions without exposing the keys. See wallets and keys.
Hash
A fixed-length fingerprint produced from any data. A tiny change in the data gives a completely different hash. Each block contains the hash of the previous block.
K
Key
See private key and public key.
L
L1
Layer 1. An independent blockchain such as Bitcoin, Ethereum or Solana. See layer 1, layer 2 and layer 3.
L2
Layer 2. A network that runs on top of a layer 1 to make transactions cheaper, and posts data to it. Arbitrum, Base and OP Mainnet are examples.
L3
Layer 3. A chain that settles on a layer 2 rather than on Ethereum directly, often built for one application.
L1 data fee
The part of a layer 2 fee that pays for posting your transaction's data to Ethereum. See rollups.
Lamport
The smallest unit of SOL. One SOL equals 1,000,000,000 lamports. Solana priority fees are quoted in micro-lamports, millionths of a lamport.
Liquidity
How easily an asset can be traded or moved without changing its price much. Also, the tokens held in a pool or bridge that make swaps and transfers possible.
M
Mempool
The waiting area of pending transactions that have been sent but are not yet in a block. See Bitcoin mempool and fees.
Miner
A participant in a proof of work network who competes to add the next block and earns a reward.
N
Native token
The main coin of a chain, used to pay fees. Examples are ETH on Ethereum, BTC on Bitcoin and SOL on Solana.
Node
A computer that runs a blockchain's software, stores the chain and checks the rules.
Nonce
On Ethereum-style chains, a counter on each address that numbers its transactions in order, which stops a transaction from being replayed. In Bitcoin mining, a number miners change while searching for a valid block.
O
Optimistic rollup
A layer 2 that assumes batches are correct unless challenged during a window of about 7 days. See optimistic vs zk rollups.
P
POL
The gas token of Polygon PoS, formerly called MATIC. See Polygon.
Priority fee
An extra payment to the block producer that makes a transaction more attractive to include sooner. On Ethereum it is also called a tip. On Solana it is priced per compute unit.
Private key
A secret number that proves you control an address and lets you sign transactions. Never share it. See wallets and keys.
Proof of stake
A consensus method where validators lock up coins as a deposit and can lose part of it if they break the rules. Used by Ethereum and Solana.
Proof of work
A consensus method where miners spend computing power to solve a puzzle for the right to add a block. Used by Bitcoin.
Public key
A value calculated from a private key that can be shared safely. Addresses are derived from it.
R
Reorg
Short for reorganization. When a chain replaces its latest few blocks with a different version. Rare for older blocks.
Rollup
A layer 2 that executes transactions off the main chain and posts the data to it. See rollups.
RPC endpoint
A web address of a node that apps and wallets use to read from and send to a blockchain. See public RPC endpoints.
S
Sat/vB
Satoshis per virtual byte. The unit for Bitcoin fee rates. See Bitcoin mempool and fees.
Satoshi
The smallest unit of Bitcoin. One bitcoin equals 100,000,000 satoshis.
Seed phrase
A list of 12 or 24 words that can regenerate all the private keys in a wallet. Keep it offline and never share it. See wallets and keys.
Self-custodial wallet
A wallet where you hold the private keys yourself and nobody else can move your funds.
Sequencer
The server that orders and executes transactions on a rollup and gives quick confirmations. See rollups.
Sidechain
A separate blockchain with its own validators, connected to another chain by a bridge. Polygon PoS is a sidechain. See layer 1, layer 2 and layer 3.
Signature
Proof, created with a private key, that the owner approved a transaction.
Slashing
A penalty in proof of stake where a validator loses part of its deposit for breaking the rules.
Slot
A fixed time period in which one block can be produced. Ethereum slots last 12 seconds and Solana slots about 400 milliseconds.
Smart contract
A program on a blockchain that runs as written when someone calls it. See what is a blockchain.
SPL token
The token standard on Solana. USDC on Solana is an SPL token. See stablecoins.
Stablecoin
A token designed to keep a steady value, usually one US dollar. See stablecoins.
Staking
Locking coins as a deposit to help secure a proof of stake network, in return for rewards.
T
Tip
Another name for the priority fee on Ethereum-style chains.
Token
A balance managed by a smart contract on a chain, as opposed to the chain's native coin. USDC is an example.
TPS
Transactions per second. A measure of how many transactions a chain processes. The number depends on what is counted, so compare it with care.
Transaction
A signed instruction to a blockchain, such as sending funds or calling a contract. See read a transaction.
Transaction hash
The unique ID of a transaction. You can paste it in the ChainMonitor search to find it.
U
USDC.e
A bridged version of USDC found on some chains such as Polygon and Arbitrum, as opposed to native USDC. See stablecoins.
UTXO
Unspent transaction output. The coin-like pieces that make up a Bitcoin balance. Each can be spent only once. See what is a blockchain.
V
Validator
A participant in a proof of stake network that proposes and approves blocks.
Validity proof
A mathematical proof that a rollup batch follows the rules. Used by zk rollups.
Virtual byte
Also vByte or vB. A unit of transaction size on Bitcoin, equal to four weight units. Fee rates are quoted per virtual byte.
W
Wallet
An app or device that holds your keys and lets you sign transactions. It does not store coins itself. See wallets and keys.
Wei
The smallest unit of ether. One ETH equals 10^18 wei.
Weight unit
Bitcoin's measure of block capacity. A block holds up to 4 million weight units.
Wrapped token
A token that represents another asset from a different chain, backed by funds locked by a bridge. See bridges and their risks.
Z
Zero-knowledge proof
A way to prove something is true without revealing the underlying data. Used in zk rollups.
zk rollup
A layer 2 that posts a validity proof with each batch, so withdrawals need no long challenge window. See rollups.