Diffusion is the technical name for the process in which particles or molecules of different substances get mixed up due to molecular movement. The particles are in continuous random motion, they have high kinetic energy and collide with each other numerous times. After some time the particles get inter mixed due to the high velocity and random movement of the particles.
i) mixing of water and ink without any stirring.
ii) diffusion of perfume in air, the smell of which reaches you through air in a short time.
iii) A helium balloon deflates a little bit every day as helium diffuses through the balloon.