Q:

Is diffuse reflection same as Rayleigh scattering?

Is there a fundamental difference in their definitions?

A:

Diffuse reflection is very common. In fact, we can see most of the objects around us due to the diffuse reflection. This is because most surfaces tend to scatter incident light, i.e., reflect it in different directions, making such surfaces visible from different angles.
There are different scattering mechanisms contributing to the diffuse reflection, mostly having to do with the roughness of the surface and various micro-structures (crystals, fibers, etc.), causing multiple reflections at their boundaries, taking place beneath the surface. The reflected angle for each individual reflection is equal to the incident angle, but, due to random orientation of reflecting surfaces, the light is reflected in all directions.
Rayleigh scattering is a particular type of scattering by particles much smaller than the wavelength. The most familiar example of it is scattering of the blue light by air molecules, making sky visible and blue. The light gets diffused, because the direction of the photons scattered by air molecules is random.
So, although both diffuse reflection and Rayleigh scattering result in a diffused light, the underlying mechanisms are different.    

