Nonce

A nonce is a number used once: an arbitrary value a miner changes over and over while searching for a block hash that meets the network's difficulty target. Because a hash function's output cannot be predicted, the only way to find a valid block is trial and error, so miners adjust the nonce, hash the block header, check the result and repeat billions of times per second. The winning nonce is published in the block, letting anyone verify the work with a single hash computation. The word also appears in crypto with a second meaning: an account nonce counts the transactions sent from an Ethereum address, orders them and prevents replays, so context matters.