A blockchain is a growing number of records (databases), called blocks, that are connected and secured using cryptographic techniques.
Each block typically contains the cryptographic hash of the previous block, timestamp, and transaction data. By design, blockchains are resistant to data modification. Blockchain is an open distributed ledger that can efficiently and record transactions between two parties in a verifiable and permanent manner. For its use as a distributed ledger, blockchain is usually managed by a peer-to-peer network collectively following a specific protocol for communication between nodes and confirming new blocks. Once recorded, the data in a block cannot be changed retroactively without changes to subsequent blocks, which requires a majority network consensus.
Blockchain is designed from the ground up to be secure (secure by design) and is an example of a distributed computing system with high Byzantine Fault Tolerance (BFT). Decentralized consensus can be reached with blockchain. This makes blockchain suitable for recording events, medical records, and other record management activities, such as identity management, transaction processing, evidence documentation, food traceability, and voting.
The blockchain was created by Satoshi Nakamoto in 2008 and is used as a ledger for public transactions of the Bitcoin cryptocurrency. The invention of the blockchain for bitcoin made it the first digital currency to solve the problem of double-spending without requiring a trusted or centralized authority. This bitcoin design has also inspired other applications.