Blockchain
A distributed, append-only database where each new block of records cryptographically links to the previous one. Powers cryptocurrencies like Bitcoin.
A blockchain is a distributed digital ledger that records transactions across many computers in a way designed to be tamper-evident. Each "block" of records is cryptographically linked to the previous one, forming a chain that becomes increasingly difficult to alter as it grows longer.
Blockchain is the underlying technology of cryptocurrencies like Bitcoin and Ethereum, but it has uses beyond finance: supply chain tracking, digital identity, smart contracts, and decentralised applications. The defining feature is trustless consensus — many parties agree on a shared record without needing a central authority.
Different blockchains use different mechanisms to achieve consensus. Proof of Work (used by Bitcoin) requires miners to solve cryptographic puzzles, consuming significant energy. Proof of Stake (used by Ethereum since 2022) requires validators to put up collateral, dramatically reducing energy use.