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pseudoprime: n.

A backgammon prime (six consecutive occupied points) with one point missing. This term is an esoteric pun derived from number theory: a number that passes a certain kind of “primality test” may be called a pseudoprime (all primes pass any such test, but so do some composite numbers), and any number that passes several is, in some sense, almost certainly prime. The hacker backgammon usage stems from the idea that a pseudoprime is almost as good as a prime: it will do the same job unless you are unlucky.