A blockchain is an open, distributed ledger that can record transactions between two parties in a verifiable and permanent way. The ledger itself can also be programmed to trigger transactions automatically.
A blockchain – originally block chain – is a distributed ledger that is used to maintain a continuously growing list of records, called blocks. Each block contains a timestamp and a link to a previous block. A blockchain is typically managed by a peer-to-peer network collectively adhering to a protocol for validating new blocks. By design, blockchains are inherently resistant to modification of the data. Once recorded, the data in any given block cannot be altered retroactively without the alteration of all subsequent blocks and the collusion of the network. Functionally, a blockchain can serve as "an open, distributed ledger that can record transactions between two parties in a verifiable and permanent way." The ledger itself can also be programmed to trigger transactions automatically.
Blockchains are secure by design and are an example of a distributed computing system with high Byzantine fault tolerance. Decentralized consensus can therefore be achieved with a blockchain. This makes blockchains potentially suitable for the recording of events, medical records, and other records management activities, identity management, transaction processing, and documenting provenance.
Definition taken from Wikipedia - Blockchain
One of the most well-known use-cases is cryptocurrencies, but can be used for distributed agreement on other things.
