A blockchain is a decentralized, distributed digital ledger that is used to record transactions across many computers so that the record cannot be altered retroactively without the alteration of all subsequent blocks and the consensus of the network.

In other words, a blockchain is a database that is shared across a network of computers. Each block in the chain contains a record of multiple transactions, and once a block is added to the chain, the data it contains cannot be altered. The decentralized nature of a blockchain means that it is not controlled by any single entity, and the network is secured by cryptography. This makes it a transparent and secure way to store and transfer data.

Blockchains are used for a variety of purposes, including recording financial transactions and tracking the movement of goods and assets. They have the potential to revolutionize many different industries and have already been used to create new types of digital currencies, such as Bitcoin.