Nonce

A value used in security protocols that is never repeated with the same key. For example, challenges used in challenge-response authentication protocols generally must not be repeated until authentication keys are changed, or there is a possibility of a replay attack. Using a nonce as a challenge is a different requirement than a random challenge, because a nonce is not necessarily unpredictable.

In cryptography, a time-variant parameter, such as a counter or a time stamp, that is used in key management protocols to prevent message replay and other types of attacks. [After X9.42]