Blockchain

A blockchain is a shared, append-only ledger in which transactions are grouped into blocks, and each block includes a cryptographic hash of the previous one. That chaining links blocks in order, so altering an old record would change its hash and break every block that follows. Copies of the ledger are maintained by many independent computers, or nodes, which agree on its contents through a consensus mechanism.

Why it matters

Because no single party holds the only copy and the history is cryptographically linked, a blockchain lets participants who do not fully trust each other agree on a common record without a central intermediary. This underpins cryptocurrencies and other applications. It also means the security of the ledger depends on how decentralised the nodes are and on the strength of the consensus mechanism protecting it. Public blockchains are typically permissionless and transparent, meaning anyone can join and inspect the history, while private or permissioned ledgers restrict participation and trade some of that openness for control.

Common misunderstanding

Blockchains are often called unhackable or immutable in an absolute sense. Records are extremely hard to change once buried under later blocks, but the guarantee is economic and probabilistic, not magical. A network with too few participants can be attacked, and the applications built on top can still contain bugs or fraud. The ledger’s integrity also says nothing about whether the data written to it was truthful in the first place; a blockchain faithfully records both honest and misleading entries.

Related terms

See consensus mechanism, node, proof of work and proof of stake.