Define ‘Block Header’ in the Context of a Blockchain

A block header is a crucial data structure at the beginning of every block in a blockchain. It contains metadata about the block, including the previous block's hash, a timestamp, the Merkle root of all transactions within the block, and the nonce.

In Proof-of-Work, miners compete to find a nonce that, when combined with the other header data, produces a hash below a specific target. This successful hash links the block to the chain and validates the block.