A digital system in which information is stored in the form of blocks connected to each other in a way that makes it difficult or impossible to change.

A blockchain is a distributed ledger with growing lists of records/transactions (blocks) that are securely linked together via cryptographic hashes (signatures). Each block contains a cryptographic hash of the previous block, a time-stamp, and the transaction data. The time-stamp proves that the transaction data existed when the block was created. Since each block contains information about the previous block, they effectively form a chain, with each additional block linking to the ones before it. Consequently, blockchain transactions are irreversible in that, once they are recorded, the data in any given block cannot be altered retroactively (by going back in the past) without altering all subsequent blocks.

Blockchain technology was first introduced/created by Satoshi Nakamoto in 2008 to serve as the public distributed ledger for bitcoin cryptocurrency transactions. The implementation of the blockchain within bitcoin made it the first digital currency to solve the double-spending problem without the need of a trusted authority or central server.