A power function is a function in which the independent variable occurs as a power (i.e. an index), eg 2^x,3^-4x
is this wrong?
No, that's the definition of an exponential function.
As sa-ri-ga-ma mentioned (and I will generalize what the user said), a power function has the form $\displaystyle f(x)=x^a$, where $\displaystyle a$ can be any real number.