What is a blockchain?

A shared, append-only ledger of transactions replicated across many independent computers and linked together by hashing, so mutually distrusting parties can agree on one history with no central operator.

It is really just a database with one expensive extra property: no single party controls the state. That property is what Bitcoin first used to let strangers transact peer-to-peer without a bank.

Go deeper:

• Blockchain — Wikipedia — the lead figure diagrams hash-linked blocks forming the chain.