A block chain or blockchain is a distributed database that maintains a continuously-growing list of transaction records hardened against tampering and revision. It consists of data structure blocks—which hold exclusively data in initial blockchain implementations, and both data and programs in some of the more recent implementations—with each block holding batches of individual transactions and the results of any block chain executable. Each block contains a timestamp and information linking it to a previous block.
The block chain is seen as the main technical innovation of bitcoin, where it serves as the public ledger of all bitcoin transactions. Bitcoin is peer-to-peer, every user is allowed to connect to the network, send new transactions to it, verify transactions, and create new blocks, which is why it is called permissionless. This original design has been the inspiration for other crypto currencies and distributed databases.