Definition
A block header contains information used to organize and verify blockchain history. Its exact fields depend on the network. In Bitcoin, the header includes a reference to the previous block, a commitment to the block’s transactions, a timestamp, version information and proof-of-work-related fields.
How It Works
Linking each header to a previous block creates a sequence of cryptographic references. A transaction commitment allows supporting proofs to connect particular transactions to a block without including every transaction in the proof. In proof-of-work systems, miners search for a header hash that satisfies the required target. Other consensus systems use different structures and validation rules.
Key Considerations
A header is not the complete block. Checking headers can provide useful evidence about ordering or accumulated work, but does not by itself validate every transaction or prove that all underlying data is available. Full nodes perform additional checks. A timestamp is also a protocol field with constraints rather than a universally precise record of the instant each transaction occurred.