The multifactorial is a generalisation of the factorial. The double factorial is written x!! and is defined to be (x)*(x-2)*(x-4)*…. If x is even, then the last term is 2; if x is odd, then the last term is 1. The triple factorial is written x!!! and is defined to be (x)*(x-3)*(x-6)*… (again, until you run out of positve integers).
In general, the k-multifactorial of x can be written (with Eindhoven notation) as: (* : i is an integer and 0 ≤ i < ⌊x/k⌋ : x - ik).
The product of k consecutive k-multifactorials is the factorial of the largest number.
Source: Wikipedia, http://www.wikipedia.org/wiki/Factorial