Bitcoin

Capital B: the network and protocol. Lowercase b: the currency unit (BTC).

Every payment is a transaction signed with the owner's private key. Transactions wait in the Mempool until a miner puts them into a Block. Blocks are chained together, so changing an old payment would mean redoing all the work that came after it. That chain is the Blockchain.

Blocks are produced by Mining: miners compete to find a block that meets the Proof of work target. The winner collects new coins (the subsidy) plus the fees of the transactions it included. The Difficulty adjustment keeps the average pace at one block every ten minutes, however much computing power joins.

Thousands of independent full nodes check every block against the same rules. A node rejects a block that breaks them, for example one that pays the miner too much, no matter who mined it.

New coins enter circulation only as the block subsidy. It started at 50 BTC and is cut in half every 210,000 blocks, roughly every four years (the Halving). Because of that schedule the total can never exceed 21 million BTC.

Satoshi Nakamoto published the whitepaper on 31 October 2008 and mined the Genesis block on 3 January 2009. The first person-to-person payment went to Hal Finney nine days later. In May 2010 Laszlo Hanyecz paid 10,000 BTC for two pizzas (Bitcoin Pizza Day). Later upgrades include SegWit (2017) and Taproot (2021); US spot ETFs started trading in January 2024.

By convention, capital-B Bitcoin is the network and protocol; lowercase bitcoin is the currency unit, ticker BTC. Amounts below one coin are often counted in satoshis.