blockchain
A distributed ledger that records all Bitcoin transactions in chronological, cryptographically linked blocks. Immutable and publicly verifiable by anyone.
The Bitcoin blockchain is a continuously growing list of records, called blocks, that are linked and secured using cryptography. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data, forming an unbreakable chain back to the very first block (the Genesis Block, mined in January 2009).
This structure makes the blockchain tamper-resistant: altering any historical record would require recalculating the proof-of-work for every subsequent block, an effort that exceeds the computational power of the entire rest of the network combined. This is why Bitcoin transactions, once confirmed, are considered final.
The blockchain is maintained by thousands of nodes worldwide. No single entity controls it. Every participant can independently verify every transaction ever made, without trusting any third party.