Nonce

A nonce ("number used once") is a value that can be used only one time for a particular purpose. In crypto the word has two common meanings.

In proof-of-work mining, the nonce is a 32-bit field in the block header that miners change over and over while hashing. Each new nonce gives a completely different hash, and the miner who finds one that produces a hash below the network's target wins the block. Because modern miners exhaust all four billion or so nonce values very quickly, they also vary other data, such as a field in the coinbase transaction, to get fresh attempts.

On account-based chains such as Ethereum, the nonce is a counter of how many transactions an address has sent. Each new transaction must use the next number, which stops the same signed transaction being replayed and keeps transactions in order. A "stuck" Ethereum transaction can be replaced by sending a new one with the same nonce and a higher fee.