pseudoprime

pseudoprime n.  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 was, before about 1985,
   called a `pseudoprime' (the terminology used by number theorists
   has since changed slightly; pre-1985 pseudoprimes are now
   `probable primes' and `pseudoprime' has a more restricted meaning
   in modular arithmetic).  The hacker backgammon usage stemmed 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.