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Definition: blockchain

A decentralized network architecture that continuously validates the integrity of transactions. A public blockchain uses volunteers who typically dedicate at least one computer 24/7 to perform the validation and ensure that no previous transaction has been altered.

Because all data in a computer are nothing more than a collection of bits and bytes, and they can be changed by anyone who can access the digital file. Even encrypted data can be altered if the encryption key used to scramble the data 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.

Both Network and Database
Starting with Bitcoin, a blockchain is the latest and most unusual architecture to come down the pike. It is both network and database. A blockchain is called a "decentralized network," "distributed network," "distributed database" and "distributed ledger." Tech guru David Pogue calls it a "tamperproof database."

Thousands of Copies
A blockchain is replicated on computers as few as a dozen to thousands depending on the network. For example, Bitcoin has more than 10,000 nodes operating on any given day, and each node has a copy of the entire blockchain or an abbreviated version for quicker verification. Although operating in computers around the world, a blockchain functions like a single computer but with no downtime. See blockchain sharding.

Two Significant Advantages
A blockchain is the technology behind Bitcoin, Ethereum and many other cryptocurrencies. However, because a blockchain provides a verifiable list of ownership, it is also used for recording contracts, titles and other legal documents. This irreversible guarantee of ownership is one of blockchain's major advantages.

A second and equally important feature is that blockchains eliminate the middleman such as a bank or other financial institution. As a result, the blockchain universe is known as "decentralized finance" (see DeFi).

Ethereum - General Purpose
Although Bitcoin is the most widely known, Ethereum is the second most popular blockchain. As a general-purpose and programmable platform, Ethereum is more versatile (see Ethereum and NFT).

A Type of Distributed Ledger
A blockchain is continuously maintained by invested parties, which may perform only validation or be in the business of adding new transactions to obtain fees, which is known as "mining" (see Bitcoin mining). See blockchain oracle, Bitcoin, public key cryptography, ICO, distributed ledger and cryptocurrency.

One-Way Linkage
A block is a collection of transactions, and blocks are chained together by storing a one-way summary ("hash") of the data from the header of the previous block. "Validation nodes" are constantly checking the linkage starting with the first block to the most recent. If a block were altered, a link is broken. For compensation, "mining nodes" check the format of new transactions and compete with other miners to add a new block to the blockchain (see block reward). See crypto mining, Merkle tree and cryptographic hash function.

The Block Header in Perspective
Only 80-bytes in size, the header is minuscule compared to the 500 or more transactions in the block, each at least 250 bytes long. By 2022, the full Bitcoin blockchain was approaching 400GB in size and growing several gigabytes per month.