DEFINITION of Block Header (Cryptocurrency)

A block header is used to identify a particular block on an entire blockchain and is hashed repeatedly to create proof of work for mining rewards.

BREAKING DOWN Block Header (Cryptocurrency)

blockchain consists of series of various blocks that are used to store information related to transactions that occur on a blockchain network. A block contains a unique header, and each such block is identified by its block header hash.

The header is an 80-byte long string, and is comprise of the 4-byte long Bitcoin version number, 32-byte previous block hash, 32-byte long merkle root, 4-byte long timestamp of the block, 4-byte long difficulty target for the block, and 4-byte long nonce used by miners.

As a part of a standard mining exercise, a block header is hashed repeatedly by miners by altering the nonce value. Through this exercise, they attempt to create proof of work, which helps miners get rewarded for their contributions to keep the blockchain system running. (For more, see How Does Bitcoin Mining Work?)