A blockchain is a distributed ledger that can record transactions between two parties in a verifiable and permanent way. The transactions are recorded in blocks that are linked using cryptography.
By design, a blockchain is resistant to modification of the data. Once recorded, the data in any given block cannot be altered retroactively without alteration of all subsequent blocks, which requires consensus of the network majority. Although blockchain records are not unalterable, blockchains may be considered secure by design.