What about positive real numbers? Not just 1, 2, 3, ..., but also 1.5, pi, etc.
The generalization of the factorial is the gamma function. For various reasons there's an offset of one: [itex]\Gamma(n) = (n-1)![/itex]. The gamma function is defined for complex numbers as well as real numbers. The only exception are non-positive real integers. The gamma function "blows up" at 0, -1, -2, etc.