Pseudoprime
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.
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Jargon File]
