A blockchain is a shared digital ledger where records are grouped into blocks and cryptographically linked. NIST defines it as a shared ledger with linked records. Nodes use consensus mechanisms to agree on accepted transactions.
A blockchain is a shared digital ledger maintained across a network. Records are grouped into blocks, and cryptographic links connect each block to the one before it. The resulting chain makes changes detectable. The National Institute of Standards and Technology (NIST) describes this as a shared ledger with linked records.
What enters that ledger is determined through network rules. Nodes use validation rules and consensus mechanisms to agree which transactions and blocks are accepted, reducing reliance on a single controlling intermediary. That describes a technology rather than a single application: blockchain is not synonymous with Bitcoin, though Bitcoin is a prominent blockchain-based system.
The exact sequence differs between networks, but the path from a user's instruction to a shared record follows a recognisable pattern. A user first authorises a transaction with a private key. The resulting digital signature allows the network to check that the instruction came from the holder of the relevant key, without disclosing that key.
Ethereum offers a useful proof-of-stake example. Users sign transactions with private keys; nodes verify valid ones and place them in a mempool, a holding area for pending transactions. A validator proposes a block, other validators re-execute it and attest to it, and Ethereum's consensus rules determine the canonical chain, according to Ethereum Foundation developer documentation.
Bitcoin uses a different consensus approach: proof of work. Its original design sets out a peer-to-peer process in which digitally signed transactions are broadcast, collected into blocks and secured through proof of work. As blocks are added, the chain of hashes makes rewriting older transaction history increasingly difficult, as described in Bitcoin: A Peer-to-Peer Electronic Cash System.
"Increasingly difficult" is more accurate than "impossible." The practical strength of a blockchain record depends on the network design, its participants and the economic or operational cost of attempting to replace accepted history. It also depends on what users mean by finality: some systems offer a degree of confidence that grows over time rather than an instant, irreversible settlement point.
In a blockchain, nodes, cryptography and consensus do different jobs. Nodes communicate with the network and may store, validate or help update the shared ledger. Cryptography protects information and links records: digital signatures let the network verify that a cryptocurrency transaction was approved by the relevant private-key holder, while hashes make changes to earlier blocks apparent by disrupting their links to later blocks.
Agreement requires more than those cryptographic tools. Networks use validation rules and a consensus mechanism to establish which eligible blocks and transaction history become shared. NIST's blockchain technology overview identifies proof of work, proof of stake and proof of authority among the approaches a network may use.
Proof of work secures Bitcoin's block-production process through computational work; proof of stake relies on validators within Ethereum's consensus process. The approaches involve different trade-offs, and each network sets rules for participation, verification and resolving competing versions of events. No label alone makes a ledger trustworthy.
In a permissionless blockchain, participation is generally open under the protocol's rules. People may be able to submit transactions, run nodes or take part in validation without being admitted by a central operator. This model can reduce reliance on a conventional intermediary. It does not remove trust altogether; it shifts trust towards the protocol, its implementation and the incentives and behaviour of the network participants.
A permissioned network, by contrast, restricts some combination of reading, submitting transactions, operating nodes or validating blocks. That can suit a group of known institutions that need a shared record while retaining defined access and accountability. It is still a distributed-ledger arrangement, but its governance model is materially different from an open public network.
The choice affects oversight. The Basel Committee on Banking Supervision has noted that permissionless systems may involve unknown third parties, complicating due diligence and oversight. For an organisation considering a blockchain system, the question is therefore not merely whether it is decentralised. It is who sets the rules, who can change or enforce them, who runs the infrastructure and what happens when participants disagree.
Cryptocurrencies are the best-known use of blockchain, the record-keeping model can also support supply-chain tracking, data registries, digital identity and records management. NIST lists these as potential applications of the technology. In each case, the possible value lies in coordinating a record across parties rather than simply storing information in a new format.
Consider supply-chain tracking. A producer, carrier, warehouse and retailer may each need to record a handover or status update. A shared ledger could provide a common sequence of those entries, so that authorised participants are working from the same history. The design does not prove that a package was handled at a stated temperature or arrived at a stated time; those facts still depend on the reliability of the people, devices and processes supplying the data.
Registries and records management present a similar case. A blockchain may help create an auditable trail of submissions and changes where several parties need visibility. A conventional database administered by a trusted entity may remain the better choice when access must be tightly controlled, changes need straightforward correction, or the cost and complexity of operating a distributed network outweigh the benefits of sharing control.
Depending on its design, a blockchain can make transaction information publicly visible rather than private. Visibility may assist auditability, sensitive commercial or personal data require an access model and data-handling design appropriate to that use.
A blockchain does not eliminate settlement risk or ensure immediate finality. The Bank for International Settlements identifies computational costs, public transaction visibility and probabilistic rather than immediate settlement finality as possible design-dependent trade-offs. Recording an instruction on a chain is not the same as completing its legal or financial outcome outside the network.
The Basel Committee identifies operational, cybersecurity, legal, compliance, governance and settlement-finality risks in blockchain systems. These questions extend beyond cryptography to key management, software operation, participant incentives, applicable rules, and the procedures available when an error, outage or dispute occurs.
Blockchain is best understood as a shared, rule-governed record that can help multiple parties coordinate without relying entirely on one central record keeper.
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