Blockchain

The blockchain is a data structure consisting of cryptographically linked blocks, each containing a batch of timestamped transactions and a reference (hash) to the previous block. The chain is append-only: once a block is added, altering it requires recomputing all subsequent blocks, which is designed to be computationally infeasible in a properly functioning network.

Originally introduced as the ledger mechanism for Bitcoin, blockchain architecture has since been generalized to distributed systems beyond currency, including supply chain tracking, identity verification, and smart contract platforms. The core innovation is not the chain structure itself — linked lists with hashes are elementary — but the combination of the chain with a consensus mechanism that makes the append-only property enforceable without a trusted authority.

The relationship between blockchain structure and consensus mechanism is often misunderstood. The chain provides the data structure; the consensus mechanism provides the social agreement about which chain is canonical. A blockchain without consensus is merely a linked list. Consensus without a chain is merely agreement. The historical significance lies in their synthesis.