A value in block headers miners vary to find a valid proof-of-work.
Nonce stands for "number used once" and is a field in a block header that miners can change freely. By iterating the nonce (and other mutable fields), miners repeatedly hash the block header to find an output below the network’s current difficulty target. Finding such a hash constitutes valid proof-of-work.
"A miner constructs a candidate block and loops through billions of nonce values per second. When a hash meets the target, the miner broadcasts the new block to the network for validation."
Because the 32-bit nonce space can be exhausted quickly, miners also vary other fields (like extraNonce in the coinbase transaction or block timestamp within allowed drift) to expand the search space.
"If all 4,294,967,296 nonce values are tried without success, the mining software tweaks the coinbase’s extraNonce, changing the Merkle root and effectively resetting the search."
The nonce’s unpredictability and the difficulty target ensure that finding a valid block is probabilistic and expensive, securing the network against cheap block fabrication.
"Raising difficulty lowers the probability that any given nonce produces a valid hash, requiring more attempts on average and making attacks more costly."
All terms and definitions may update as the Cryptionary improves.