Diffusion

Diffusion is the movement of particles from a higher concentrated place to a lower concentrated one. Like when your sister fart, you will smell it a mile away, that's because the fart particles will travel from a higher concentrated place(your sister's butt) to a lower concentrated one(your nostril).

Diffusion can mean the spreading movement of any particle, that means that your sisters fart isn't the only thing that can diffuse. Diffusion only stops when the concentration becomes the same in all the places.