What does the exclamation mark mean in big-o, i.e. O(X!)? [duplicate]

Question

I've seen examples of big-o expressed as O(X!) but I am not sure what this means, according to the accompanying charts it is very slow.

Can someone give an example of an O(X!) algorithm?

Thanks

Answer 1

It means factorial. It is the product of the numbers from 1 to X. So by example, 5! = 1 * 2 * 3 * 4 * 5 = 120.