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.

[Jargon File]