Nonce

A value used in security protocols that is never repeated with the same key. For example, nonces used as challenges in challengeresponse authentication protocols generally must not be repeated until authentication keys are changed. Otherwise, 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.
SOURCE: SP 800-63

A random or non-repeating value that is included in data exchanged by a protocol, usually for the purpose of guaranteeing the transmittal of live data rather than replayed data, thus detecting and protecting against replay attacks.
SOURCE: CNSSI-4009