A blockchain is a digital ledger, or list of transactions, that is shared across a distributed network of computers. It records and stores every transaction that occurs in the system in a secure and permanent way. Blockchain is used in cryptocurrency to maintain a public and immutable record of all transactions made on the network. Transactions are recorded in blocks, and each block is connected to the previous block in the chain. This creates an unbroken, tamper-proof chain of information that enables users to securely send and receive digital currencies. The blockchain also provides a level of trust and transparency not found in traditional banking systems.