Blockchain
A blockchain is a shared, tamper-resistant digital ledger that records transactions in blocks linked together in chronological order across a network of computers.
Each block contains a batch of transactions, a timestamp and a cryptographic fingerprint, called a hash, of the previous block. This chaining means that altering an old record would require recalculating every block after it and convincing most of the network to accept the change, which makes tampering extremely difficult on large networks.
Copies of the ledger are kept by many independent participants, or nodes. They agree on new blocks through a consensus mechanism. Bitcoin uses proof of work, which relies on computing power; Ethereum uses proof of stake, in which validators lock up coins as collateral.
Public blockchains such as Bitcoin and Ethereum are open for anyone to join and inspect. Private or permissioned blockchains limit access and are used by some companies for record keeping and settlement.
Beyond cryptocurrencies, blockchains support smart contracts, tokenized assets and supply chain tracking. Example: anyone can look up a bitcoin transaction on a public blockchain explorer and see the amount and the addresses involved, though not necessarily the identities behind them.