Bitcoin Glossary

An ASIC is a chip built to do one job only — compute SHA-256 for mining — and it outperforms home computers and graphics cards by millions of times.

Average block time is the mean mining time across many blocks — Bitcoin targets 10 minutes, and the real average over a year stays close to it.

A 51% attack is when someone controls most of the network's hashrate and can rewrite recent blocks or censor transactions.

A Bitcoin address is where bitcoin is sent — a unique identifier derived from a public key, similar to an account number you can rotate freely.

A block is a page of Bitcoin's ledger: a batch of confirmed transactions stamped by miners and chained to the previous page.

Block height is simply how many blocks exist before a given block — the Genesis block is height 0, the next one is 1, and so on.

The block reward is everything the winning miner collects from a block: the block subsidy plus all the transaction fees included in it.

The block subsidy is the new bitcoin created out of thin air for whoever mines a block; it started at 50 and halves every 210,000 blocks.

Block time is the actual minutes and seconds between one block and the next — on average about 10 minutes, but each individual block varies.

Block validation is the checklist every node runs on a received block — proof of work, transactions, no double-spends — before accepting it as true.

Bech32 is the encoding used by SegWit and Taproot addresses starting with "bc1": all lowercase, free of confusing characters, with an error-detecting checksum.

A BIP is a numbered Bitcoin Improvement Proposal: the written document where a protocol change is proposed, debated and finally recorded.

Block size is the historical 1 MB limit on Bitcoin's blocks, a rule that SegWit later reworked into a weight-based capacity.

Block weight is SegWit's capacity measure for blocks: base data counts four times as much as witness data, up to 4,000,000 units.

The blockchain is Bitcoin's ever-growing ledger: a chain of blocks where each one carries the fingerprint of the block before it.

The coinbase is the special first transaction of every block, the one through which the miner collects the block subsidy and all the fees.

Coinbase maturity is the rule that a miner must wait 100 confirmations before spending the subsidy they earned, which shields the chain from self-serving reorgs.

A chain reorganization happens when nodes switch to a longer valid chain, orphaning blocks they had already accepted as canonical.

A confirmation means one more block has been mined on top of your transaction, each adding about ten minutes of protection.

Circulating supply counts the bitcoin already mined and believed to be held — about 19.9 million coins in 2026 — and it grows with every new block.

Difficulty is how hard it is to find a valid block right now — the network raises it when more miners join so blocks stay ~10 minutes apart.

The difficulty adjustment is Bitcoin's automatic self-regulation: every 2,016 blocks it recalibrates how hard mining must be to keep blocks near 10 minutes.

A difficulty epoch is the ~2-week, 2,016-block window between difficulty adjustments — like a school term after which grades get recalculated.

A double spend is the attempt to spend the same bitcoin twice, the very problem that Bitcoin's blockchain exists to prevent.

Dust is a tiny bitcoin output that would cost more to spend later than it is worth, so nodes reject it and wallets warn you.

The extra nonce is an extra counter miners embed in the coinbase transaction to keep generating fresh hashes after the regular nonce runs out.

The Genesis block is block 0, mined by Satoshi Nakamoto on 3 January 2009, whose 50 bitcoin reward can never be spent and which hides a newspaper headline.

The halving is the event, roughly every four years, when the block subsidy is cut exactly in half — from 50 down to today's 3.125 bitcoin.

A hash function is a one-way machine turning any data into a fixed-length fingerprint: the same input gives the same output, a tiny change gives a totally different one.

Hashrate is the total computing power securing Bitcoin — how many billion guesses per second miners make trying to find the next block.

A hard fork changes the rules so drastically that old nodes reject the new blocks — it can split the chain into two separate coins.

A miner is any person or company running hardware that competes to create the next block and get its reward.

Mining is the process of bundling transactions into a new block by solving a mathematical puzzle — it secures the network and issues new bitcoin.

A mining pool is a team of miners who combine their hashrate and split the rewards proportionally instead of competing against each other.

The mempool is each node's waiting room for unconfirmed transactions, where they sit until a miner packs them into a block.

The Merkle root is a single fingerprint that summarizes every single transaction in a block, and it lives in the block header.

Multisig is a spending rule that requires several signatures — say two out of three keys — before the funds can actually move.

A millibitcoin is one thousandth of a bitcoin — 0.001 BTC, or 100,000 sats — a handier size for everyday amounts.

The nonce is the counter inside a block header that miners change millions of times per second trying to beat the target.

A node is any computer running Bitcoin software that follows the rules, checks blocks and passes transactions along to other nodes.

Proof of work shows a miner burned real electricity to find a hash below a target, making blocks costly to create and history costly to rewrite.

The priority fee is the tip you add above the minimum rate to nudge miners into including your transaction sooner.

SHA-256 is the hash function Bitcoin uses: it scrambles any input into a fixed 256-bit fingerprint that never changes if the input doesn't.

Solo mining means competing for blocks alone with your own hardware — all-or-nothing: you keep the whole reward if you win, and get nothing if you don't.

Stratum is the protocol mining pools use to send work to machines and collect partial results back, the industry standard since around 2012.

Schnorr signatures are a cleaner signing scheme enabled by Taproot: smaller, faster, and able to combine many signatures into a single one.

Script is Bitcoin's tiny programming language: the set of rules attached to each output that says who is allowed to spend it next.

SegWit is the 2017 soft fork that moved digital signatures out of the main transaction data, freeing space and fixing transaction malleability.

A soft fork tightens the rules so that old nodes still accept the new blocks — a backward-compatible way to upgrade Bitcoin.

A stale block is a valid block that lost the mining race — another block at the same height ended up with more work behind it.

Sat per vbyte is the standard fee-rate unit: how many satoshis you pay for each virtual byte your transaction occupies.

A satoshi is the smallest unit of bitcoin — one hundred millionth of a coin, or 0.00000001 BTC — and the yardstick fees are measured in.

Bitcoin's supply cap is the hard-coded limit of 21,000,000 coins that can ever exist, written into the rules everyone runs.

The target is the number a block's fingerprint must fall below for the block to be valid — the smaller it is, the harder mining is.

A transaction fee is a small voluntary payment to the miner, quoted in sat/vB, that gets your transaction included in a block.

Taproot is the 2021 upgrade that made complex bitcoin spends simpler, cheaper and more private by combining Schnorr signatures with MAST.

The time warp attack fakes block timestamps around difficulty adjustments to trick the network into keeping mining artificially easy.

A block's timestamp records roughly when it was mined, and consensus rules keep those claimed times honest and close to real time.

A transaction is a signed message that moves bitcoin from one set of outputs to another, recorded publicly on the blockchain.

The txid is a unique label for a transaction — a fingerprint computed from its contents, used to look it up on any explorer.

A full node downloads and checks every block and transaction from the very first one, enforcing Bitcoin's rules on its own instead of trusting others.

The fee market is the auction for scarce block space: users bid fees and miners pack the highest-paying transactions first.

Finality is the point where reversing a Bitcoin transaction becomes practically impossible, with each confirmation making it exponentially harder.

Locktime is a value in a transaction that stops it from being added to the blockchain before a chosen block height or time.

An orphan block is a valid block whose parent isn't known yet, so it waits aside until the missing block finally arrives.

Replace-by-fee is an opt-in rule that lets you resubmit an unconfirmed transaction with a higher fee so it confirms sooner.

A UTXO is an unspent transaction output: bitcoin actually lives in these discrete chunks, and your balance is simply the sum of the ones you can spend.

In Bitcoin, the witness is the signature part of a SegWit transaction, moved out of the base data so blocks can hold more.

Bitcoin's inflation rate is the yearly share of new coins added to the existing stock — under 1% now and halved again every four years.