DEFINITION
A digital money and payment network with no company, government, or bank in charge — secured by cryptography and a fixed supply of 21 million coins.
Launched in January 2009 by the pseudonymous Satoshi Nakamoto, Bitcoin solved a problem thought unsolvable: transferring value online without a trusted middleman. Every transaction is recorded on a public ledger (the blockchain) maintained by thousands of independent computers.
Capital-B Bitcoin usually means the network and protocol; lowercase bitcoin (or BTC) means the currency unit. Its fixed 21-million supply is the property everything else — the halving, the mining industry, the "digital gold" thesis — flows from.