Abbreviation for “number only used once.” A nonce is an arbitrary number that is used only once in a cryptographic message to ensure that old communications cannot be reused in replay attacks. In blockchain, it refers to the first number a blockchain miner needs to discover before solving for a block in the blockchain. Miners use computing power to guess the nonce as part of Proof of Work, and the miner who is able to find the value is awarded the block and paid a reward.