Blockchain

A blockchain is a record of transactions grouped into blocks, where each block contains a cryptographic hash (a kind of fingerprint) of the block before it. Because every block points to its predecessor, changing an old transaction would change its hash and break every block that follows, which makes tampering easy to detect.

Public blockchains such as Bitcoin and Ethereum are maintained by many independent computers rather than one company. These computers follow shared rules, called a consensus mechanism, to agree which new block is valid. Proof of work and proof of stake are the two most common.

A blockchain is not automatically secure or useful: its security depends on how many independent participants maintain it and how costly it is to attack. "Private" or "permissioned" blockchains run by a single firm or consortium give up much of that independence in exchange for speed and control.