What is leibniz notation?

The notation of the derivative of a function f, as d(f(x)/dx or dy/dx where y=f(x) is known as the Leibnitz notation. Some books say that it is not the ratio of dy and dx, which is in fact, truly speaking, not true. The quantities dy and dx are known as the differential of y and differential of x, and their ratio in that order represents the derivative function wherever it exists. So, this notation is extremely flexible.