What is a blockchain?

A blockchain is a distributed digital ledger whose validated records are grouped into blocks and cryptographically linked to prior history. Network participants apply shared protocol rules to proposed updates and use a consensus mechanism to decide which valid history to follow. The design makes unauthorized historical changes detectable and, under the network's operating assumptions, increasingly difficult—not universally impossible. NISTIR 8202: Blockchain Technology Overview

A blockchain is therefore more than a database containing hashes. It combines a data structure, independently executable validation rules, replicated state and a process for resolving competing histories.

Why use a blockchain instead of one database?

A conventional database can be efficient when one trusted administrator is authorized to order and edit records. A blockchain addresses a different coordination problem: participants maintain compatible copies and can verify updates without giving one database operator sole authority over the accepted history.

That trade-off is not free. Replication and consensus add computation, communication, storage and governance complexity. Permissionless networks distribute participation under protocol rules; permissioned networks restrict particular roles to approved entities. “Blockchain” does not mean that every participant has identical access or that no organization influences the system. NISTIR 8202, sections 2–4

Four layers that maintain the ledger

Transactions or state updates

Users or applications submit signed transactions or other protocol-defined updates. Nodes check authorization, format, balances, nonces, spending conditions and other applicable rules. A submitted message is not valid merely because it was broadcast.

Blocks and cryptographic commitments

Valid updates are grouped into blocks. A block normally commits to its parent and contents through hashes or tree roots. If earlier data changes, later commitments no longer match unless the affected history is rebuilt. This provides tamper evidence; it does not by itself decide which competing history the network accepts.

Replication and independent validation

Nodes exchange blocks and maintain data needed for their roles. A full validating node checks rules itself rather than trusting a block explorer or another node's label. Not every network participant stores all history or performs the same duties, and “distributed” does not imply that resources or influence are evenly distributed.

Consensus and fork choice

Consensus coordinates the accepted order among valid proposals. Bitcoin full nodes validate blocks, while proof of work supplies the chain-selection signal based on accumulated work. Ethereum proof of stake uses validators, attestations, fork choice and checkpoint finality. Consensus mechanisms make different assumptions and should not be collapsed into the word “mining.” Bitcoin Developer Guide: block chain; Ethereum: proof-of-stake

What changes when a transaction is accepted?

Bitcoin uses an unspent transaction output (UTXO) model. A valid transaction consumes existing outputs under their spending conditions and creates new outputs; nodes reject conflicting spends in the accepted history. Ethereum maintains account-based state and executes transactions to update balances, nonces, contract code or storage under protocol rules.

In either model, the blockchain is not a folder that moves coin files between addresses. Nodes compute which state follows from the accepted sequence of valid updates. A wallet prepares and signs instructions or interacts with a provider; the wallet interface is not the shared ledger itself.

Tamper evidence, reorganizations and finality

Cryptographic linking makes a changed historical block inconsistent with later commitments, but hashes do not physically prevent rewriting. Resistance comes from the cost and decision rules required to replace the accepted history, together with honest validation by participants.

Recent history can be reorganized when fork choice selects another valid branch. Some protocols provide probabilistic confidence as more blocks accumulate; others define explicit finality through votes or certificates. Finality assumptions can fail under severe faults, protocol bugs or threshold compromises. Describing every blockchain record as “permanently immutable” hides these conditions.

What consensus does not prove

Consensus can establish that network participants accepted data under protocol rules. It cannot independently prove that an external statement is true. If an oracle reports a temperature or a system records that a parcel arrived, putting that value onchain proves the record and its processing history—not the physical event itself. NIST discusses this boundary as a challenge in linking physical and digital records. NISTIR 8202, sections 7.1 and 7.3

The same distinction applies to code. Deterministic execution can reproduce what a smart contract was instructed to do, but it does not guarantee that the code has no defect, that an interface explained the call correctly or that the economic outcome is desirable.

Blockchain, cryptocurrency and distributed ledger

A blockchain is one distributed-ledger design that groups records into linked blocks. A cryptocurrency is an asset accounted for under a network's rules. A distributed ledger technology (DLT) is a broader category and does not always use a single chain of blocks. Networks may also use DAGs or other structures, so not every distributed ledger should be renamed a blockchain.

A blockchain can support purposes beyond transferable assets, but using the structure does not automatically make an application decentralized, private, scalable or trustworthy. Those properties require separate evidence about governance, node operation, data visibility and protocol design.

FAQ

Does every blockchain use mining?

No. Mining is associated with proof-of-work systems. Proof-of-stake networks use validators and votes, while permissioned systems may use other consensus protocols.

Is a blockchain the same as one block?

No. A block is one protocol-defined data unit. A blockchain is the linked ledger history maintained under network rules.

Can data ever be removed from a blockchain?

The answer depends on what “removed” means and on the protocol. A reorganization can displace recent blocks; applications may stop displaying data; some systems permit state changes or pruning. Historical commitments and copies held by participants may remain. “Delete,” “prune,” “revert” and “become non-canonical” are different operations.

Related concepts and further reading

Read about blocks, blockchain addresses, crypto wallets and gas fees, or return to the Crypto Glossary.

For a broader historical comparison, see OKX Learn's overview of blockchains, Bitcoin and Ethereum. It is supplementary reading rather than a protocol specification.

Sources

• NISTIR 8202: Blockchain Technology Overview — distributed-ledger architecture, consensus, permission models, tamper resistance and external-data limitations.
• Bitcoin Developer Guide: block chain — full-node validation, UTXOs, proof of work and chain selection.
• Bitcoin Developer Reference: block chain — block and transaction data structures.
• Ethereum: proof-of-stake — validators, attestations, fork choice and finality.
• Ethereum: accounts — account-based state, EOAs and contract accounts.