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The Free On-line Dictionary of Computing (foldoc)

A backgammon prime (six consecutive occupied points) with one
point missing. This term is an esoteric pun derived from a
mathematical method that, rather than determining precisely
whether a number is prime (has no divisors), uses a
statistical technique to decide whether the number is
"probably" prime. A number that passes this test is called a
pseudoprime. The hacker backgammon usage stems from the idea
that a pseudoprime is almost as good as a prime: it does the
job of a prime until proven otherwise, and that probably won't
happen.

[{Jargon File}]