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Definition: blockchain

A decentralized network architecture that validates the integrity of digital data. A blockchain ensures that any alteration to a stored transaction is quickly detected. Because all data in a computer are nothing more than a collection of bits and bytes, they can be changed by anyone who can access the digital file. Even encrypted data can be altered if the private key is exposed. However, all transactions in a blockchain are "linked" together (see illustration below), and a change to any existing transaction breaks a link in the chain.

Thousands of Copies
Decentralization is at the heart of the blockchain. Blockchains are continuously verified from the very first transaction to the last by hundreds or even thousands of nodes, depending on the network, and any broken link is identified right away.

In addition, blockchain nodes have a complete copy of all transactions from the very first to the last. There is no chance of downtime in a blockchain network if one or more nodes fail to operate.

For Bitcoin and Other Data
A blockchain is the underlying architecture of Bitcoin. However, blockchains are used for other cryptocurrencies, and because it provides a verifiable list of ownership, it is also used for contracts, fundraising and the recording of legal documents. Both the public and private sectors are using or are exploring blockchain technologies for various purposes. The incentive is undeniable proof of digital authenticity without a middleman such as a bank or other financial institution.

Ethereum - General Purpose
Although Bitcoin is the most widely known, Ethereum is the second most popular blockchain. As a general-purpose platform, sometimes called the "world computer," Ethereum is actually a much more comprehensive and more complex blockchain than Bitcoin (see Ethereum and non-fungible token).

A Type of Distributed Ledger
A blockchain is replicated on computers as few as a dozen to thousands depending on the size of the network. It is continuously maintained by invested parties, which may perform only verification or be in the business of recording the transactions to obtain fees (see Bitcoin mining). See blockchain oracle, Bitcoin, public key cryptography, ICO, distributed ledger and cryptocurrency.

A Blockchain Is One-Way Linkage
Blocks are chained together by storing the hash of the header of the previous block. If a transaction within a block were altered, the link to the subsequent block would be broken. The Merkle root is a hashed summary of transactions within the block (see Merkle tree). See cryptographic hash function.