Nonce is short for “number used once”, and a nonce is the number that allows miners to find a valid Proof-of-Work. While mining, miners look for a hash of their block that is valid. Because a hash is deterministically generated from the input data, hashing the same block will always yield the same hash. To generate unique hashes, hoping to find a valid one while avoiding changing the block itself, miners use a nonce, a number they can increment simply to change the hash of their block.