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]