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.