Blockchain

The blockchain is the public ledger of every Bitcoin transaction, organized as a sequence of blocks, each one cryptographically tied to the one before it. It's the data structure that makes Bitcoin work.

The construction is simple. Every block contains the SHA-256 hash of the previous block's header in its own header. So block 800,000 references block 799,999 by hash; block 799,999 references 799,998; and so on back to the genesis block. If you change a single byte of any historical block, its hash changes, which means every block after it now references the wrong predecessor, which means every subsequent block needs to be re-mined. With Bitcoin's proof-of-work burden, that gets impossibly expensive fast.

Every full Bitcoin node stores the entire chain - currently ~600 GB and growing by ~150 GB/year - and independently validates every block against every consensus rule. There is no master ledger; there are tens of thousands of identical copies, each verified against the others. To rewrite Bitcoin's history, you'd have to convince a majority of those nodes to accept your fake version. They won't.

The word "blockchain" has been generalized to mean any append-only hash-linked data structure, and the technology has been bolted onto countless projects of varying credibility. Bitcoin's blockchain is the one that matters: the first, the largest, the most secure, and the only one that actually solves the problem it was designed for.

See the Mining rabbit hole for why the chain is so hard to rewrite, and Block for what's inside each link.