(also cryptographic nonce)
In cryptography, a nonce is a random number that is only used once by an authentication protocol in a particular communication. Nonces are commonly employed to thwart replay attacks.
The term “nonce” is an abbreviation of “number only used once.” Nonce values are typically created using random number generators (or other algorithms that are able to generate unique values based on various system parameters).
In a replay attack, the attacker intercepts a valid message and reuses it to impersonate the legitimate user. Adding a nonce to each message helps prevent these attacks — if the hackers try to replay an intercepted message, the receiving system can recognize the nonce and automatically repel the attempt.