What is a nonce in blockchain?

Nonce is short for “number used once.” Blockchains use the word in two different ways, and beginners often meet both.

1. The mining nonce (bitcoin)

Each bitcoin block header has a small field called the nonce. Miners change it, hash the header, and check whether the result is below the network’s target. If not, they change it again. Billions of tries later, someone finds a winner and the block is accepted. That trial-and-error is proof of work. See what is a hash function.

2. The account nonce (Ethereum)

On Ethereum, every account has a counter. Your first transaction has nonce 0, the next has 1, and so on. The network processes them in order, and the same nonce cannot be used twice. That stops a signed transaction from being replayed.

Why it shows up in your wallet

If an Ethereum transaction is stuck, later ones wait behind it. Some wallets let you “speed up” or “cancel” by sending a new transaction with the same nonce and a higher fee. Only one of the two can confirm.