Wikipedia also has a good discussion of the basics as well as some generalizations: http://en.m.wikipedia.org/wiki/Fractional_derivative http://en.m.wikipedia.

Wikipedia also has a good discussion of the basics as well as some generalizations: http://en.m.wikipedia.org/wiki/Fractional_derivative http://en.m.wikipedia.org/wiki/Fractional_derivative Turns out there are even some applications, albeit rather esoteric ones.