Blockchain
A shared digital ledger where transactions are recorded in linked blocks that are extremely difficult to alter.
A blockchain is a shared digital ledger — a record of transactions — maintained by many computers at once instead of a single company or bank. Transactions are grouped into blocks, and each new block contains a cryptographic fingerprint of the one before it, forming a chain back to the very first block.
That chained structure is what makes blockchains hard to tamper with: changing an old transaction would require redoing every block after it, across thousands of independent computers, at the same time. Bitcoin’s blockchain (launched 2009) was the first; today thousands exist with different designs and trade-offs between speed, cost and decentralization.