Blockchain

A blockchain is a shared digital ledger in which transactions are grouped into blocks, and each new block is cryptographically linked to the previous one, forming an unbroken chain of records. Because every participant in the network holds a copy of the same ledger and new blocks require network-wide agreement, altering past entries is practically impossible without controlling most of the network.

The design solves a hard problem: how can strangers agree on a single history of transactions without trusting a central bookkeeper? Bitcoin's blockchain, for example, has recorded every BTC transaction since January 2009; anyone can download the full history and independently verify that the rules were followed. If someone tried to rewrite an old block, its cryptographic fingerprint would change and every later block would no longer match, so the tampering would be instantly obvious.

A common misconception is that blockchain and Bitcoin are the same thing; Bitcoin is one application of blockchain technology, while other blockchains like Ethereum run smart contracts and entirely different assets. Blockchains vary widely in design — some are public and open to anyone, others are permissioned and used privately by companies — but all share the core idea of an append-only ledger secured by cryptography and consensus.