A blockchain is a publicly auditable ledger in which transactions are grouped into cryptographically linked blocks and agreed upon by a decentralized network of nodes running proof-of-work consensus.

Because every full node keeps its own copy of the chain and verifies each block, there is no central point of failure. Altering a past transaction would require re-doing the proof-of-work for that block and all subsequent blocks, then convincing a majority of the network to accept the change—an attack considered economically infeasible.

Key properties include:
• Decentralized - anyone can run a node; no single party controls the history.
• Immutable - once confirmed, a transaction cannot be edited or deleted.
• Transparent yet pseudonymous - all Bitcoin transactions are visible to anyone, while user identities are represented only by addresses.

By solving the double-spend problem without a trusted intermediary, the blockchain enables peer-to-peer electronic cash and lays the foundation for innovations such as the Lightning Network and tokenization layers like Liquid.