According to Wikipedia, A blockchain is a continuously growing list of records, called blocks, which are linked and secured using cryptography.  By design, a blockchain is inherently resistant to modification of the data.  It is "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way.  Once recorded, the data in any given block cannot be altered retroactively without the alteration of all subsequent blocks, which requires collusion of the network majority.