Definition
A block header is a unique identifier that provides information about a block. It contains the metadata, the time and difficulty level of creating a new block, the Merkle roots of transactions, and the nonce.
Understanding the term
An ordered sequence of blocks begins with the genesis block, where each block header contains three distinct sets of information, including the previous block hash, the version number, timestamp of the block, the block’s difficulty target, Merkle root, and a nonce.
Bitcoin uses the 80-byte format to serialize its block header and hash it with the proof-of-work algorithm. Typical transactions are 250 bytes, whereas block headers are 80 bytes and average blocks contain over 500 transactions. Thereby, the total size of a complete block, including all transactions, is 1,000 times greater than the total size of the block header.
As part of mining, miners change the nonce value to hash a block header. This helps maintain a highly functioning and efficient blockchain system by building proof of work. Besides being excellent for mining, block headers are also useful for light clients because of their relatively small size.
Takeaway
Block headers are unique identifiers that are cryptographically secure and immutable. They are used in identifying specific blocks and are hashed repeatedly to generate proof-of-work rewards. Compared to full blocks, they can be easily processed and sent across blockchain networks.