Nonce is shorthand for 'Number Used Once' and is a number that is consumed in an action and not recorded. In this case, the true 'number' is the full block header which is hashed as a single string however the 'Nonce' field is the only part which changes during process of hashing a given block header.
The "nonce" in a Bitcoin block header is a 32-bit (4-byte) field, whose value is adjusted by Miners during the Proof of Work process in an effort to generate a block hash less than or equal to, the current target of the network. The rest of the fields, with the exception of the timestamp, may not be changed, as they have a defined meaning.
Any change to the block data (such as the nonce) will make the block hash completely different. Since it is believed infeasible to predict which combination of bits will result in the right hash, many different nonce values are tried, and the hash is recomputed for each value until a hash less than or equal to the current target of the network is found. The target required is also represented as the difficulty, where a higher difficulty represents a lower target. As this iterative calculation requires time and resources, the presentation of a block with a valid nonce value constitutes proof of work.
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