(Number ONCE) An arbitrary number that is generated to provide a unique identification or for security purposes such as when logging in to a network (see initialization vector). The nonce is used only once and not repeated. Although random and pseudo-random numbers theoretically produce unique numbers, there is the possibility that the same number can be generated more than once. However, if a very large, true random number is used, the chances are extremely small. A perfect nonce is the time of day; for example, 12.53 seconds past 5:13pm on 1/18/2012 can only occur once.