A nonce is an abbreviation for “number only used once,” which is a number added to a hashed—or encrypted—block in a blockchain that, when rehashed, meets the difficulty level restrictions. The nonce is the number that blockchain miners are solving for. When the solution is found, the blockchain miners are offered cryptocurrency in exchange.(https://www.investopedia.com/terms/n/nonce.asp)
Example of Block and Nonce
The average number of hashing attempts is automatically adjusted by the protocol to ensure that each new block is generated – on average – every 10 minutes. This process is known as difficulty adjustment and is what determines the mining threshold (i.e., how many zeros the block hash needs to start with to be considered valid). The difficulty in mining a new block is related to the amount of hashing power (hash rate) committed to a blockchain system. The more hashing power dedicated to the network, the higher the threshold will be, meaning that more computational power will be required to be a competitive and successful miner. In contrast, if miners decide to stop mining, the difficulty will be adjusted, and the threshold will drop, so less hashing power will be required to mine, but the protocol will make sure that the block generation follows a 10-minute schedule, regardless.(https://academy.binance.com/en/glossary/nonce)
How are Blockchain Nonces used in PoW?
In Bitcoin’s PoW mining process, the goal is to solve a mathematical puzzle in order to discover the next block hash and receive Bitcoin rewards.
Blocks on the Blockchain are identified by their header, which consists of the Merkle Root, timestamp, the previous block hash, and a nonce. The nonce is the only field in the header that isn’t predetermined.
Miners must find a nonce value that, when plugged into the hashing algorithm, generates a hash value that is lower than the target difficulty. It can be said that the nonce is the missing piece of the puzzle needed to discover the next block and miners receive the block reward.
So basically, mining revolves around brute-forcing the nonce through a mathematical algorithm and finding that lucky number that will reward the miners for their efforts. (https://paybis.com/blog/what-is-a-blockchain-nonce/)