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Nonce means 'for the present time' or 'for a single occasion or purpose', although the word is not often found in general use. A dictionary may note nonce words, those for which there is only a single textual instance.
In security engineering, a nonce is a 'number used once'. It is often a random or pseudo-random number issued in an authentication protocol to ensure that old communications cannot be reused in 'replay attacks'.
Nonce is used in HTTP Digest Authentication to calculate an MD5 digest of the password. The Nonce is different each time the 401 authentication challenge response is presented, thus making the replay attack virtually impossible.
In the UK, the term nonce is a slang word used to refer to a paedophile, as in 'James is a nonce'. It is a joke on Never Mind the Buzzcocks that Bill Bailey resembles a 'Klingon nonce'. It was originally used by Her Majesty's Prison Service in the UK for prisoners that were not to be mixed with the general population for their own safety. The acronym stands for Not On Normal Courtyard Exercise.
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