A number that stands for one of many possible actual values, typically in examples ("%6.2f gives 123.45"), to FakeIt or just to compile code before some computation is implemented. The use of "special" numbers makes it clear that the value is made up and doesn't represent actual data.
Typical metasyntactic numbers include:
* 42 (from HitchhikersGuideToTheGalaxy)
* 23 (from TheIlluminatusTrilogy ''or from ThePrincipiaDiscordia - which came first, the chicken or the chao'')
* 666, 69, 13, 17, and any other number with a well-known meaning  (What does 17 mean?)
* as many identical (111111) and consecutive (1234.567) digits as required

In software, arbitrary "interesting" numbers (also including powers of 2, powers of 10, prime numbers) are used in many places:
* size of an array or buffer
* size of a HashTable
* enumerated constants
but they are not metasyntactic numbers since they are an estimate or a perfectly good value.

-- LorenzoGatti