What is the Bitcoin blockchain?

Detailed explanation

The blockchain is the ordered transaction history organized in linked blocks. Each block header contains the previous header's hash. This creates a verifiable sequence: if an earlier block changes, subsequent hash references no longer match.

An important clarification: there is no central database that all nodes blindly access. Each full node downloads blocks, checks them against its consensus rules and stores its own validated view of the chain. When nodes apply the same valid rules and know the same chain with the most cumulative work, their best chains usually agree.

The blockchain contains confirmed transactions. Unconfirmed transactions may be in local mempools and are not yet part of the chain. The blockchain contains no private keys either. It is publicly verifiable but is not synonymous with all of Bitcoin: the system also includes nodes, the network protocol, consensus rules, wallets, miners and users choosing software and creating transactions. The blockchain is thus a core dataset of the system, not the entire system.