Blockchain

A blockchain is a ledger built as a chain of blocks, each containing a batch of transactions and a cryptographic hash of the block before it, so that altering any historical record would break every link that follows. Bitcoin's blockchain has grown block by block, roughly every 10 minutes, since its first block on January 3, 2009.

Why it matters

The chained structure, combined with proof of work and thousands of independent nodes holding full copies, produces something new: a public record that no single party administers and no participant can quietly rewrite. Changing a past bitcoin transaction would require redoing the accumulated computational work from that point forward against the entire network's ongoing output, which becomes economically absurd within a few blocks. This is what allows strangers to transact final settlement without trusting an intermediary. The word has since been applied to countless projects, but the security properties depend on the surrounding system, decentralized validation and costly block production, not on the data structure alone.

In the gold vs bitcoin debate

The blockchain gives bitcoin an auditability gold cannot match: anyone with a computer can verify the entire supply and every transaction in history, while auditing gold means physically assaying bars in vaults, and the total above-ground stock is an estimate. The trade-off runs the other way for privacy, since gold changes hands without records while bitcoin's ledger is permanent and public. Verifiable supply against recorded history is one of the cleanest trade-offs in the whole comparison. The ledger's openness is also what makes the 21 million cap enforceable, since every node checks every block against the issuance rules.