In the context of blockchain technology, a nonce is a number that miners must solve in order to create a new block. Each nonce is unique to the block it is part of, and its primary purpose is to add security and prevent replay attacks. When miners find a correct nonce value that, when hashed with the block data, produces a hash which meets the required difficulty level, they can add the block to the blockchain. The term is derived from 'number used once'.