Stands for “number used once”. A 32-bit number in a block header which is iterated during a search for proof-of-work. It is an otherwise meaningless number which is used to alter the outcome of a hash. A nonce is used to try and produce a digest that fits the numerical parameters set by the Bitcoin difficulty. A different nonce will be used with each hashing attempt, meaning that billions of nonces are generated when attempting to hash each transaction block. Each time Bitcoin hashes a block, it increments a nonce within the block it is trying to verify. If the numeric value of the effectively random hash is below a certain amount determined by the block generation difficulty, then the block is accepted by other clients and is added to the chain. Using a nonce ensures that the hash calculation is cryptographically secure (cannot be hacked) since it cannot be repeated without that random number. Each time the nonce is changed, the hash of the block header is recalculated. If nonce overflows before valid proof-of-work is found, an extra nonce is incremented and placed in the coinbase script. Alternatively, one may change a merkle tree of transactions or a timestamp .
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc
Number Used Once or Number Once. A nonce, in information technology, is a number generated for a specific use, such as session authentication. Typically, a nonce is some value that varies with time, although a very large random number is sometimes used. In general usage, nonce means “for the immediate occasion” or “for now.” In the case of Blockchain Proof of Work scenarios, the hash value, found by a Miner, matching the network’s Difficulty thus proving the Block Validity is called Nonce as well.