What Is a “Nonce” in the Context of Proof-of-Work Mining?

A "nonce" (number only used once) is a variable number that a miner adjusts in the block header to find a valid hash. Miners repeatedly change the nonce and re-hash the block data until the resulting hash is below the network's current difficulty target.

The process of rapidly trying billions of nonces per second is the "work" in Proof-of-Work. The first miner to find a valid nonce-hash combination wins the block reward.