A blockchain is a shared digital ledger maintained simultaneously across many independent computers. Transactions are grouped into "blocks", and each block is cryptographically linked to the one before it, forming a chain. Because every participant holds a copy and the links are tamper-evident, altering a past record would mean overpowering the whole network at once — impractical on a large chain.
This design removes the need for a single trusted middleman to keep the books. Instead of one bank's database being the authority, the network itself agrees on what is true through its consensus rules. That property is what lets strangers transact without knowing or trusting each other.
Blockchains are not only for payments. They can run programmable logic — smart contracts — which underpins decentralised finance, tokens and much else. The trade-off is that a fully decentralised chain is slower and costlier per transaction than an ordinary centralised database.
Worked example
Bitcoin's blockchain has recorded every transaction since 2009 in a single continuous chain that anyone can inspect.
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This definition is general education, not investment advice. Markets — especially crypto — are volatile and you can lose money. Please read our disclaimer and see our methodology.