! Factorial

One-argument form  See also two-argument form Binomial

When applied to a positive whole number, ! gives the product of the whole numbers from 1 to that number, inclusive.

             ! 3                     (Equivalent of 1×2×3)
       6

If the argument is non-integer and positive, ! gives the mathematical 'gamma function' of the argument + 1.

             ! 2.5
       3.32335097