Nonce definitions

nŏns
The present or particular occasion.
noun
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The present use, occasion, or time; time being.
noun
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Denoting something occurring once.
adjective
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The one or single occasion; the present reason or purpose (now only in for the nonce).

That will do for the nonce, but we'll need a better answer for the long term.

noun
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(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.Pronounced like the "nons" in "nonsense," nonce is actually an English word that means "for the present occasion or time."
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(lexicography) A nonce word.

I had thought that the term was a nonce, but it seems as if it's been picked up by other authors.

noun
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(computing) A number, usually generated randomly or from the time, used once in a cryptographic protocol, to prevent replay attacks.
noun
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(UK, slang, pejorative) A sex offender, especially of children; a paedophile.

That bloke who lives at number 53 is a nonce!

noun
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(UK, slang) A stupid or worthless person.
noun
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(cryptography) A datum constructed so as to be unique to a particular message in a stream, in order to prevent replay attacks.

In this protocol we use the serial number of the message as a nonce.

noun
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(cryptography) In a security engineering context, a value used only once.
noun
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Origin of nonce

From Middle English for the nones for the occasion alteration of for then anes for for for then neuter dative sing. of the the1 ones, anes once once