Define the term: diffusion?

Hint: As we know that the diffusion means spread. When we cook something delicious, we smell good and we want to have that due to that smell. So the smell which comes to us has reason.

Complete step by step answer:
As we know that matter has three states, solid state, liquid state and gaseous state. In solid state, the particles of solid are very closely packed and have a strong force of attraction between them. Therefore, the particles of solid cannot move means particles do not have more kinetic energy so the mixing of particles into the surrounding is not possible.
But when we talk about the liquid state of matter the particles of liquid are not very closely packed and have less force of attraction between them as compared to solid. So the particles of liquid have tendency to move means particles have some kinetic energy so the mixing of particles into two or more liquids to form homogeneous solution is possible such as if we put a few drops ink in a beaker containing water after some time we will see the ink particles diffuses through water and the whole solution is now blue.
Now, coming on the gaseous state of matter, the particles of gases are very far from each other and have negligible force of attraction between them. Therefore, the particles of gases have much tendency to move means particles have very much kinetic energy so the intermixing of particles into surrounding to form homogeneous mixture.
The diffusion is defined as the process of intermixing of molecules of two or more substances to form a homogeneous solution. For example, on spraying a perfume in a room we are able to smell it right till the other end. This happens because the particles of the perfume diffuse into the air present around us.

Note:
According to Graham’s law of diffusion, the rates of diffusion of gases are inversely proportional to the square roots of their molecular masses.