What is the opposite of exponential growth?
If by opposite, you mean inverse :
The function that reverses the x-y values of the exponential function is the natural logarithmic function ; f(x) = ln x.
As the exponential function begins with a slow increase and progresses towards infinite growth, so the natural logarithmic function increases rapidly and then slows towards a zero rate of change.
To see this on a coordinate grid, reflect the exponential function in the line y = x.
image courtesy of Wikipedia