Blockchain

A record-keeping system deliberately designed so that no single company, bank, or government gets to be the one official keeper of the ledger.

Cheat Sheet

A blockchain is a digital record-keeping system that stores data in a chain of linked "blocks," distributed across many computers rather than held by a single central authority.
Once a block is added to a blockchain and confirmed, altering its contents is extremely difficult without also changing every subsequent block — making blockchain records highly resistant to tampering.
Blockchain is the underlying technology behind cryptocurrencies like Bitcoin, but the underlying record-keeping concept has been explored for many other uses beyond digital currency, including supply chain tracking and digital identity.
Most public blockchains rely on a decentralized network of independent computers, called nodes, that collectively verify and agree on the accuracy of new transactions before they're added.
"Smart contracts" are self-executing pieces of code stored on certain blockchains (notably Ethereum) that automatically carry out predefined actions when specific conditions are met.
Blockchain technology has faced significant criticism over energy consumption (particularly for early proof-of-work systems), scalability limitations, and a large gap between ambitious proposed use cases and proven, widely adopted real-world applications.

The 60-Second Version

A blockchain is a digital record-keeping system that stores data in a chain of linked "blocks," distributed across many computers rather than held by a single central authority. Once a block is added to a blockchain and confirmed, altering its contents is extremely difficult without also changing every subsequent block, making blockchain records highly resistant to tampering. Blockchain is the underlying technology behind cryptocurrencies like Bitcoin, but the underlying record-keeping concept has been explored for many other uses beyond digital currency, including supply chain tracking and digital identity. Most public blockchains rely on a decentralized network of independent computers, called nodes, that collectively verify and agree on the accuracy of new transactions before they're added. "Smart contracts" are self-executing pieces of code stored on certain blockchains, notably Ethereum, that automatically carry out predefined actions when specific conditions are met. Blockchain technology has faced significant criticism over energy consumption, particularly for early proof-of-work systems, scalability limitations, and a large gap between ambitious proposed use cases and proven, widely adopted real-world applications.

The Long Version

A Ledger With No Single Owner

At its core, a blockchain is a shared digital ledger, a record of transactions or data, that's distributed across a network of independent computers rather than stored and controlled by one central institution like a bank. Every participating computer holds a copy of the same record, and changes must be collectively verified across the network rather than approved by a single central gatekeeper.

Why Tampering Is So Difficult

Each block in a blockchain contains a cryptographic reference to the block before it, meaning altering data in an already-confirmed block would also require recalculating every single subsequent block in the chain, across a majority of the distributed network simultaneously, a practically infeasible task on any established blockchain with enough participants, which is precisely what gives blockchain records their strong tamper-resistance.

Beyond Cryptocurrency

While blockchain first reached mainstream attention as the technology underlying Bitcoin and other cryptocurrencies, the underlying concept, a decentralized, tamper-resistant shared record, has been explored for a range of other potential applications, including tracking goods through complex supply chains, managing digital identity credentials, and recording ownership of digital assets, though many of these proposed uses remain far earlier in real-world adoption than cryptocurrency itself.

Smart Contracts and the Criticism Blockchain Still Faces

Some blockchains, most notably Ethereum, support smart contracts, self-executing code stored directly on the blockchain that automatically carries out predefined actions once specified conditions are met, without requiring a trusted intermediary. Despite this expanding functionality, blockchain technology continues to face substantial criticism, including the significant energy consumption associated with older verification methods like proof of work, ongoing scalability limitations, and a persistent gap between blockchain's ambitious proposed use cases and the relatively narrow set that have actually achieved widespread, proven real-world adoption.

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Glossary

Block: A batch of recorded data or transactions added as a single unit to a blockchain.
Node: An individual computer participating in a blockchain network, helping verify and store its data.
Decentralization: The distribution of control across many independent participants rather than a single central authority.
Smart contract: Self-executing code stored on a blockchain that automatically performs actions when specified conditions are met.
Proof of work: A method some blockchains use to verify transactions, requiring computers to solve energy-intensive computational puzzles.