Diffusion

Diffusion is the movement of molecules from a region of a higher concentration to a region of lower concentration until the molecules are evenly distributed.

From the definition we can deduce that there are two regions involved. You can be able to notice the movement if you are able to differentiate the two regions.

When you put water in a beaker and you add more water into that same beaker, you can never study the movement in the water unless the substance you are adding to the water has a different colour.

This is what is known as the concentration gradient. Diffusion can never happen when the concentration gradients of both regions are the same. This brings us to the definition of concentration gradient.

Concentration gradient is the difference between the concentrations of a substance (molecules or atoms) in one region compared to another region.