A target hash is the largest numeric number that a blockchain will allow as the header hash for a block. Nodes must discover a nonce (a number that can only be used once) that gives an acceptable hash for the block's header when paired with the other inputs when mining blocks using the Proof of Work consensus process. To be successful, the header hash of a block must be less than or equal to the target hash. Target hashes are often very low values beginning with many zeros. Blockchains require miners to fulfil specified hash requirements to verify that nodes have expended enough computational power to validate a block. The higher the block difficulty, the lower the target hash.
