Nonce is an abbreviation for number only used once.
This is a number that is written into the block header and must meet certain conditions (the block hash must be in the correct range).
The nonce is the number that blockchain miners are solving for. To successfully verify a block, a miner needs to be the first to guess the block’s nonce.
If the miner’s hash meets the requirements set, then the block is added to the blockchain. Searching for a solution in order to guess the nonce is referred to as proof of work. The miner who is able to find the value fastest gets the block reward.