Nonce
A per-account counter ensuring transactions execute in order and can't be replayed — and a mining variable in Bitcoin.
In account-based chains like Ethereum, a nonce is a sequential counter attached to each transaction from an address: the first transaction is nonce 0, the next nonce 1, and so on. It guarantees ordering (transactions must process in sequence) and prevents replay — the same signed transaction can’t be broadcast twice.
The nonce explains some common frustrations: a stuck transaction with a low nonce blocks all later ones behind it, which is why “speeding up” or replacing a pending transaction (rebroadcasting the same nonce with a higher fee) is a standard fix. In Bitcoin mining, “nonce” means something different — the value miners vary to search for a valid block hash.