What Is the Role of the “Nonce” in the Proof-of-Work Hashing Process?

The "nonce" (number only used once) is a variable number included in the block header that miners iterate through to change the block's hash. Since the block's content (transactions, previous block hash, etc.) is fixed, the miner must change the nonce until the resulting hash is less than the current difficulty target.

The process of rapidly trying different nonce values until a valid hash is found is the core of the Proof-of-Work mining process. The valid nonce is then included in the final, accepted block.