Blockchain

A blockchain is a distributed digital ledger that records transactions in blocks, each cryptographically linked to the one before it by including a hash of the prior block, so that altering any past entry would require rewriting every block after it across every copy of the ledger. The pseudonymous creator or creators writing as Satoshi Nakamoto introduced the concept in a 2008 white paper describing Bitcoin, and the Bitcoin network that launched in January 2009 was the first working implementation, using the blockchain to let strangers agree on a shared transaction history without a central authority. The technique was later generalized well beyond currency, most notably by the Ethereum platform launched in 2015, which added programmable smart contracts to the same block-linking structure, and blockchain-based systems are now used for applications ranging from payments to supply-chain tracking to digital identity.