double factorial (math.) with "m!! Indicates. When m is a Nature number, it means the product of all positive integer numbers not exceeding m and having the same parity with m.

 Example:
3!!!!! = 1 * 3 = 3
5!!!!! = 1 * 3 * 5 = 15
6!!!!! = 2 * 4 * 6 = 48
Eight!!!!! = 2 * 4 * 6 * 8 = 384
Another 0!! = 1!!!!! =1