What is Ethereum transaction Nonce?

[CryptoGirl]

What is Ethereum transaction Nonce?
In Ethereum, every transaction has a nonce. The nonce is the number of transactions sent from a given address.

Each time you send a transaction, the nonce incremented by 1.

    Transactions must be in Order: You cannot have a transaction with a nonce of 1 mined before one with a nonce of 0.

    No Skipping! 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.

Why?

This field prevents double-spends as the nonce is the order the transactions go in. In order for a double-spend to be "successful", you typically...

    Send a transaction to one party

    Wait for it to register

    Collect something in return from this first transaction

    Quickly send another transaction with a high gas price

    Have this second transaction mined first, therefore invalidating the first transaction.

This is why exchanges wait for you to have a certain number of confirmations before allowing you to trade freshly-deposited funds.