A decentralized ledger that stores transactions in chronological blocks, making them immutable.
Example
When you send Bitcoin to a friend, the transaction is written into a 'block' and confirmed by thousands of computers worldwide. Once in the chain, it cannot be altered or deleted — that makes the blockchain tamper-proof, with no bank in between. Anyone can trace the transaction in a block explorer. This combination of decentralization and immutability is the foundation of every cryptocurrency.