What is a Blockchain?
A blockchain is a shared digital ledger that records every Bitcoin transaction ever made. Anyone in the world can view and verify it, but no one can edit or erase past entries. Thousands of computers store identical copies of this ledger, so it does not depend on any single server or company.
The name comes from the structure. Transactions are grouped into blocks, and each block links to the one before it, forming a chain. That chain stretches back to Bitcoin's first block, created on January 3, 2009. Every block since then remains intact and publicly visible.
Why It Matters
The blockchain lets Bitcoin work without a bank, government, or central authority. In traditional finance, you trust your bank to keep an accurate balance. If the bank makes an error or freezes your account, you have to wait for them to fix it. With Bitcoin, thousands of independent computers each hold their own copy of the ledger and continuously cross-check each other. No single party controls the record.
You can verify every transaction yourself. Every entry is public, and the full history is available to anyone running a node. When you send bitcoin, that payment lands in the ledger permanently. Anyone can confirm it happened.
The blockchain also solves double-spending, a problem that blocked earlier attempts at digital money. Before Bitcoin, you had no way to prove a digital coin had not already been spent elsewhere. Once a Bitcoin transaction is confirmed and buried under additional blocks, reversing it would require more computing power than the rest of the network combined. That cost makes fraud economically pointless.
How It Works
Roughly every 10 minutes, miners compete to solve a mathematical puzzle. The winner assembles the next block of valid transactions and broadcasts it to every other computer on the network. Those computers check the work and add the block to their own copy of the chain. This process is called proof of work.
Each block contains a cryptographic fingerprint (a hash) of the block before it. That hash is the link. If someone tried to alter a transaction buried 100 blocks deep, they would need to recalculate the hash for that block and every block after it, all while the rest of the network keeps adding new ones. The energy and hardware cost makes tampering uneconomical.
You can run a full node yourself. A full node downloads and verifies the entire blockchain from block zero. It checks that coins are not spent twice, that no bitcoin is created outside the issuance schedule, and that every digital signature is valid. Thousands of people run full nodes, and each one independently enforces the rules. You can explore Bitcoin's chain activity in real time on Bitcoin Pulse.