What does 5.5! mean? Why does it equal 287.825.... ?

5.5! reads as "five point five factorial." I don't know what it means for non-whole numbers to be factorial, but I know that when you have a whole number factorial then you multiply all the whole numbers less than that number by the number. (i.e.: 6! = 6x5x4x3x2x1 = 720)

Thanks but I know what 5! is.
5.5! is included as one of the standard functions on this site as shown in my attachment and it works on this site's calculator, but, I have never seen it before and it does not work on my standard CASIO calculator. I was hoping that a site manager (or someone else) could explain it to me.