Blockchain
A distributed ledger that records transactions in chronologically linked blocks, secured by cryptography.
A blockchain is a shared, immutable ledger maintained by a decentralised network of computers (nodes). Transactions are bundled into blocks, each of which contains a cryptographic hash of the previous block — forming a chain that makes altering historical records computationally infeasible.
Because no single party controls the ledger, blockchains can enable trustless interactions between participants who don't know each other. The first large-scale implementation was Bitcoin, launched in 2009; since then thousands of blockchains have been built for purposes ranging from smart-contract execution to supply-chain tracking.
Public blockchains are open to anyone to read and write; permissioned (private) blockchains restrict participation to known entities. The trade-off between openness, performance, and security is often called the blockchain trilemma.