Nonce Meaning

In the context of Proof of Work (PoW) mining, a Nonce ("Number only used once") is an arbitrary 32-bit integer that miners modify repeatedly. The miner takes the block data, adds a nonce, hashes it, and checks if the result meets the network's difficulty target. If it doesn't, they change the nonce (e.g., from 1 to 2) and try again.

They do this trillions of times per second. Finding the correct nonce is the "Work" in Proof of Work.

In the context of Ethereum Transactions, a nonce serves a different purpose. It is a counter associated with a wallet address that tracks how many transactions that address has sent.

The first transaction is Nonce 0, the next is Nonce 1, etc. This transaction nonce is critical for security because it prevents Replay Attacks.

If you send 1 ETH to Bob, a hacker cannot take that signed transaction and broadcast it again to make you pay Bob a second time. The network sees that Nonce X has already been used and rejects the duplicate transaction.