Glossary
Nonce
A counter used to prevent replays or to search for a valid block hash — meaning depends on context (per-account on Ethereum, per-block in PoW mining).
"Nonce" means different things in different parts of a blockchain. On a PoW chain like Bitcoin, the nonce is a 32-bit field in the block header that miners increment while hashing, looking for a result below the difficulty target.
On Ethereum, every account has a transaction nonce — a counter that starts at zero and increases by one with every transaction the account sends. The nonce makes each transaction unique, which is what prevents the same signed transaction from being mined twice.