Source : Free On-Line Dictionary of Computing

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}]