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A batch of transactions chained to the block before it. One is found about every 10 minutes by whichever miner first solves a hard puzzle. Each block carries the hash of the one before, so changing an old block would mean redoing every block after it.

A block's place in the chain: how many blocks came before it. The first block, the genesis block, is block 0.

A payment written down: which earlier coins it spends (its inputs) and where the value goes (its outputs). Once it is in a block it is part of the record for good.

Each block on top of the one holding your transaction is one more confirmation. Six or more is final for most purposes: undoing it would take more computing power than anyone has.

A waiting room for transactions that have been broadcast but not yet put in a block. When it is crowded, people outbid each other with fees; when it is quiet, the minimum fee gets you in.

What you pay per unit of block space. Miners fill blocks with the best-paying transactions first, so the rate you pay, not the amount you send, decides how soon you confirm.

A new version of a waiting transaction that spends the same coins with a higher fee, so it takes the old one's place in the mempool. Most wallets call it "speed up" or "bump fee". It is not a second payment: once either version is in a block, the other can never go through.

A way to rescue a stuck transaction from the receiving end: spend one of its outputs right away with a high fee. A miner can only take the child together with its parent, so the two are judged by their combined fee rate.

The smallest unit of bitcoin: one hundred-millionth of a bitcoin (0.00000001 BTC). Named after Bitcoin's creator, Satoshi Nakamoto. People often say sats.

How much block space a transaction takes. Signatures in SegWit transactions count for a quarter, so a virtual byte is a little less than a byte on disk. A block holds about a million.

A computer racing to find the next block by guessing numbers until one gives a hash low enough. The winner collects the new coins and the fees in that block.

Miners who share their work and split the rewards, so each gets a steady income instead of a rare jackpot. A pool names itself in the coinbase of the blocks it finds: a claim, not proof.

The first transaction in every block, which pays the miner the new coins and the block's fees. It has no inputs, and miners write a short note in it, often their pool's name.

How much computing power is working to find the next block. One EH/s is a quintillion guesses a second. More hashrate makes the network costlier to attack.

How hard it is to find a block. Every 2,016 blocks (about two weeks) the network adjusts it, so that one block arrives about every 10 minutes no matter how much hashrate joins or leaves.

Every 210,000 blocks (about four years) the new bitcoin paid for each block is cut in half. It started at 50 BTC in 2009 and is 3.125 BTC since April 2024. It is why there will never be more than 21 million bitcoin.

A computer running Bitcoin software that checks every rule on its own and keeps a full copy of the chain. Anyone can run one. Ours answers every lookup on this site.

Where bitcoin can be sent, like an account number you can make as many of as you like. Addresses are public: reusing one links your payments together, so wallets make a new one each time.

An upgrade active since November 2021 (block 709,632). Taproot addresses start with bc1p; they make complex spending rules look like an ordinary payment and cost less.

An upgrade active since August 2017 (block 481,824) that moved signatures into a separate part of the transaction. It fixed a long-standing bug and made room for more transactions per block. SegWit addresses start with bc1q.