How Is ‘Nonce’ Used in the PoW Mining Process?

The nonce, or 'number only used once,' is a variable in the block header that miners iterate through to find a valid block hash. Miners repeatedly change the nonce and re-hash the block header data until the resulting hash meets the network's current difficulty target (e.g. starts with a certain number of zeros).

Finding the correct nonce is the "work" in Proof-of-Work, which proves the computational effort was expended.