Blockchain

A blockchain is best understood as a ledger — a running list of who sent what to whom — except no single bank or company controls it. Instead, thousands of computers around the world each hold an identical copy, and they continuously check one another for discrepancies. This design makes quietly altering old records extraordinarily difficult, because a change on one copy would instantly conflict with every other copy on the network.

Transactions are grouped into "blocks," and each block contains a mathematical fingerprint (called a hash) of the block that came before it. That chain of fingerprints is where the name comes from. Tamper with one block and every fingerprint after it breaks, which the network's other participants can immediately detect. You can explore how this underpins digital currencies in the blockchain explained guide.

A common confusion is treating "blockchain" and "cryptocurrency" as the same thing. Blockchain is the underlying record-keeping technology; cryptocurrencies like Bitcoin are one application built on top of it. Blockchains can also record things other than currency transfers — contracts, ownership records, and more — though in market data contexts the term almost always refers to the ledgers that power crypto assets.