Nonce
In Ethereum, every transaction has a nonce. The nonce is the number of transactions sent from a given addressAn address is a cryptographic key that "owns" bitcoins. This... More. A nonce is a number that can only be used once. In cryptographyCryptography is the encryption mechanism used to secure cryp... More, a nonce is a one-time code selected in a random or pseudo-random manner that is used to securely transmit a main password, preventing replay attacks.
Each time you send a transaction, the nonce value increases by 1. There are rules about what transactions are considered valid transactions, and the nonce is used to enforce some of these rules.
Specifically: Transactions must be in order. You cannot have a transaction with a nonce of 1 mined before one with a nonce of 0. You cannot have a transaction with a nonce of 2 mined if you have not already sent transactions with a nonce of 1 and 0.
This value prevents most cases of double-spending.