Bitcoin has more than its share of jargon, some of it technical, some of it slang, a little of it deliberately obscure. This glossary defines 111 terms in plain English. Type in the box to filter, or press / anywhere on the site to search everything at once.
All terms, A to Z
- 51% attack
- An attack in which someone controlling most of the network’s mining power rewrites recent blocks, typically to spend the same coins twice. It cannot steal coins from other people’s addresses or create coins beyond the schedule, and on bitcoin it would cost billions in hardware and power.
- Address
- A string of letters and numbers you give someone so they can pay you. It encodes the lock on a payment, usually a hash of a public key. Modern addresses start with
bc1. - Air gap
- Keeping a signing device physically disconnected from the internet, passing transactions across by QR code or memory card.
- Altcoin
- Any cryptocurrency other than bitcoin.
- ASIC
- Application-specific integrated circuit. A chip that does one thing; in bitcoin’s case, compute SHA-256 hashes far more efficiently than any general-purpose computer.
- Bech32
- The address format introduced with SegWit, beginning
bc1q. Taproot uses a revised version, bech32m, beginningbc1p. - BIP
- Bitcoin Improvement Proposal. A numbered design document proposing a change or standard. A BIP is a suggestion; it becomes real only if people run software that implements it.
- BIP-360
- A 2026 proposal for a quantum-resistant output type, pay-to-Merkle-root, that hides public keys until coins are spent through a script.
- BIP-39
- The standard that turns wallet entropy into a list of 12 or 24 ordinary words from a fixed list of 2,048.
- Bitcoin Core
- The most widely used software for running a bitcoin node, descended directly from Satoshi’s original program and maintained by a rotating group of volunteers and sponsored developers.
- Bitcoin Knots
- An alternative node client maintained by Luke Dashjr, based on Bitcoin Core with additional options and stricter default filters on data-carrying transactions.
- Block
- A batch of transactions bundled with a header and added to the chain, on average once every ten minutes.
- Block explorer
- A website that lets you look up any block, transaction or address on the public chain, such as mempool.space.
- Block height
- A block’s position in the chain, counting from the genesis block at height zero.
- Block reward
- What a miner earns for a block: the subsidy of newly created coins plus the fees paid by the transactions inside.
- Block size war
- The 2015 to 2017 dispute over whether to raise bitcoin’s block size limit. It ended with the limit kept, SegWit activated and Bitcoin Cash forked off.
- Block subsidy
- The newly created bitcoin in each block. It began at 50, halves every 210,000 blocks, and is 3.125 BTC until 2028.
- Blockchain
- A ledger stored as a chain of blocks, each containing the hash of the one before, so that changing old data visibly breaks everything after it.
- BTC
- The common ticker for bitcoin. XBT is also used in some financial systems.
- Change output
- The part of a transaction that sends leftover coins back to the spender, because inputs must be spent whole.
- Coinbase transaction
- The first transaction in each block, which creates the block reward. Not to be confused with the company of the same name.
- CoinJoin
- A privacy technique in which several people combine their payments into one transaction so an observer cannot tell whose coins went where.
- Cold storage
- Keeping private keys on a device that is never connected to the internet.
- Confirmation
- A transaction has one confirmation when it is included in a block, and one more for each block added on top. Six is the traditional standard for large payments.
- Consensus rules
- The rules every node enforces independently: the supply schedule, valid signatures, block size and so on. Breaking them gets a block rejected no matter who mined it.
- Covenant
- A proposed kind of spending condition that restricts where coins can go next, not just who can spend them. OP_CTV and OP_CAT are the best-known proposals.
- Creation and redemption
- The process by which ETF shares are made or destroyed in exchange for cash or bitcoin, expanding and shrinking the fund to meet demand.
- Custodian
- A company that holds private keys on someone else’s behalf, such as an exchange or the firm that stores an ETF’s coins.
- Cypherpunk
- A member of the 1990s movement that advocated strong cryptography as a tool for privacy and individual freedom.
- DCA
- Dollar-cost averaging. Buying a fixed amount at regular intervals regardless of price.
- Difficulty
- A measure of how hard it is to find a valid block. It rises and falls with the network’s total mining power.
- Difficulty adjustment
- Every 2,016 blocks, about two weeks, the network resets difficulty so that blocks keep arriving roughly every ten minutes.
- Double spend
- Spending the same coins twice. Preventing it without a central authority is the problem bitcoin was invented to solve.
- Dust
- An amount of bitcoin so small that it costs more in fees to spend than it is worth.
- ECDSA
- The elliptic-curve signature scheme bitcoin has used since launch. Taproot added Schnorr signatures alongside it.
- ETF
- Exchange-traded fund. A fund whose shares trade on a stock exchange. A spot bitcoin ETF holds actual bitcoin; a futures ETF holds contracts.
- Exahash (EH/s)
- A quintillion hashes per second. The bitcoin network’s total has been around a thousand of these.
- Fee rate
- What a transaction pays per unit of block space, quoted in satoshis per virtual byte (sat/vB). Miners generally pick the highest fee rates first.
- Fiat money
- Government-issued currency that is not backed by a commodity, such as dollars, euros or yen.
- Fork
- A divergence in the chain or the rules. See hard fork and soft fork.
- Full node
- A computer that downloads every block and checks every rule for itself, trusting no one.
- Genesis block
- Block zero, mined by Satoshi on 3 January 2009.
- Halving
- The cut of the block subsidy by half every 210,000 blocks, roughly every four years. The next is expected in 2028.
- Hard fork
- A rule change that loosens the rules, so that old software rejects new blocks. Unless everyone upgrades, the chain splits in two.
- Hardware wallet
- A small dedicated device that stores private keys and signs transactions without exposing the keys to your computer or phone.
- Hash
- The fixed-length fingerprint a hash function produces from any input. Tiny changes to the input produce a completely different hash.
- Hashrate
- The number of hashes computed per second, by one machine or by the whole network.
- HODL
- To hold bitcoin through volatility rather than trade it. From a misspelled 2013 forum post.
- Hot wallet
- A wallet whose keys sit on an internet-connected device. Convenient, and more exposed.
- HTLC
- Hashed time-locked contract. The mechanism that lets a Lightning payment hop across several channels without anyone in the middle being able to steal it.
- In-kind
- Creating or redeeming ETF shares by handing over bitcoin itself, not cash. Permitted for US crypto ETFs since July 2025.
- Input
- The part of a transaction that points to earlier coins being spent and proves the right to spend them.
- Inscription
- Data such as an image or text embedded in a bitcoin transaction’s witness and tied to a specific satoshi by the Ordinals convention.
- KYC
- Know your customer. The identity checks that regulated exchanges must carry out.
- Layer 2
- A system built on top of bitcoin that handles transactions off the main chain while relying on it for final settlement. Lightning is the main example.
- Ledger
- A record of who owns what. Bitcoin’s is public, shared and append-only.
- Lightning invoice
- A request for a Lightning payment, usually shown as a QR code, that encodes the amount, the destination and an expiry time.
- Lightning Network
- A network of payment channels that lets people send bitcoin instantly and cheaply off-chain, settling to the blockchain only when channels open or close.
- Liquid
- A sidechain run by a federation of companies, offering faster settlement, confidential amounts and issued assets, using bitcoin pegged in from the main chain.
- Locktime
- A field that prevents a transaction from being confirmed before a given time or block height.
- Mempool
- The waiting room. Each node’s collection of valid transactions that have been broadcast but not yet included in a block.
- Merkle root
- A single hash in the block header that commits to every transaction in the block.
- Miner
- A person or company running machines that compete to add the next block, earning the block reward if they succeed.
- Mining pool
- A service that combines many miners’ hashrate and shares out rewards, smoothing their income.
- Multisig
- A setup that requires several keys to approve a spend, for example any two of three. It removes the single point of failure of one key.
- Node
- Any computer running bitcoin software and connected to the network. See full node.
- Nonce
- A number miners change repeatedly in the block header in search of a hash below the target.
- Not your keys, not your coins
- The maxim that bitcoin held by a custodian is a claim on that custodian, not bitcoin you control.
- OP_RETURN
- A script instruction that marks an output as unspendable and lets it carry a small piece of arbitrary data. How much data nodes should relay became a heated dispute in 2025.
- Ordinals
- A convention for numbering individual satoshis in the order they were mined, which makes it possible to treat particular sats as collectibles.
- Orphan (stale) block
- A valid block that lost the race to another found at almost the same moment, and is not part of the longest chain.
- Output
- The part of a transaction that assigns an amount of bitcoin to a new lock. Unspent outputs are the coins people own.
- P2P
- Peer-to-peer. A network in which participants connect directly to each other rather than through a central server.
- Passphrase
- An optional extra secret added to a seed phrase, sometimes called the 25th word. Without it the seed opens a different, empty wallet.
- Payment channel
- A shared two-party balance, anchored by one on-chain transaction, that can be updated any number of times off-chain.
- Private key
- A secret number that lets you sign transactions and so spend coins. Whoever knows it controls the coins.
- Proof of reserves
- A cryptographic demonstration by a custodian that it actually holds the coins it owes customers.
- Proof of work
- The requirement that a block’s hash fall below a target, which can only be met by brute-force guessing. It makes blocks costly to produce and trivial to check.
- PSBT
- Partially signed bitcoin transaction. A standard file format that lets different wallets and devices cooperate on signing.
- Public key
- The number derived from a private key that others use to verify your signatures. It can be shared safely, though quantum computing complicates that for the long term.
- Quantum resistance
- The property of a signature scheme that would survive attack by a large quantum computer. Bitcoin’s current signatures do not have it; proposals such as BIP-360 aim to add it.
- RBF
- Replace-by-fee. Re-sending an unconfirmed transaction with a higher fee to get it confirmed sooner.
- Reorg
- A reorganisation: when nodes switch to a longer competing chain, discarding one or more recent blocks. One-block reorgs happen naturally; deep ones would signal an attack.
- Runes
- A protocol for issuing fungible tokens on bitcoin, launched at the 2024 halving.
- Satoshi (sat)
- The smallest unit of bitcoin: one hundred-millionth of a coin.
- Schnorr signature
- A signature scheme added by Taproot that is simpler than ECDSA and lets many signatures be combined into one.
- Script
- The small programming language used to express the conditions for spending an output.
- Security budget
- The total paid to miners in subsidy and fees, which is what makes attacking the chain expensive.
- Seed phrase
- The 12 or 24 words from which a wallet derives all of its keys. Anyone who sees it can take everything; anyone who loses it, and the device, has lost everything.
- SegWit
- Segregated Witness. The 2017 upgrade that moved signatures into a separate part of the transaction, fixing a long-standing bug and making room for more transactions and for Lightning.
- Self-custody
- Holding your own private keys rather than leaving coins with an exchange or fund.
- SHA-256
- The hash function at the centre of bitcoin, used for mining, for linking blocks and for building addresses.
- Sidechain
- A separate blockchain with its own rules to which bitcoin can be moved and later returned.
- Signet and testnet
- Parallel networks with worthless coins where developers try out changes safely.
- Soft fork
- A rule change that tightens the rules, so that old software still accepts new blocks. SegWit and Taproot were soft forks.
- Spot ETF
- A fund that holds the asset itself. For bitcoin, approved in the United States on 10 January 2024.
- Stock-to-flow
- The ratio of an asset’s existing supply to its annual new production; a rough measure of scarcity, and the basis of a much-debated price model.
- Strategic Bitcoin Reserve
- The US government holding created by executive order in March 2025 from forfeited bitcoin. As of October 2026 it had not been written into law.
- Taproot
- The November 2021 upgrade that introduced Schnorr signatures and a way to hide complex spending conditions unless they are used.
- Target
- The number a block’s hash must be below. A lower target means higher difficulty.
- Timechain
- Satoshi’s original name, in early code, for what everyone now calls the blockchain. Some bitcoiners prefer it.
- Transaction ID (txid)
- The hash that identifies a transaction.
- UTXO
- Unspent transaction output. The fundamental unit of ownership in bitcoin; your balance is the sum of the UTXOs your keys can spend.
- vByte
- Virtual byte. The unit of block space used for fee calculation since SegWit.
- Wallet
- Software or hardware that stores keys and builds transactions. It holds the keys, not the coins; the coins exist only as entries on the chain.
- Whale
- A holder with a very large amount of bitcoin.
- Whitepaper
- Satoshi’s nine-page 2008 paper describing the design.
- Witness
- The part of a SegWit transaction that holds signatures and other unlocking data.
- xpub
- Extended public key. A single string from which a wallet can generate all of its receiving addresses without being able to spend. Sharing it exposes your full history.
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Where to see these in action
Definitions only go so far. To watch a nonce being found, use the mining demo. To see inputs, outputs and a Merkle root in a real block, open the anatomy explorer. For halvings and supply, drag the slider on the economics page. For payment channels, run the Lightning animation. For fund terms such as creation, redemption and in-kind, see how a spot ETF works.