The process of molecules moving from a region of higher concentration to lower concentration until everything evens out. When you open a bottle of perfume in a room, at first, the scent is strong near the bottle, but after a while, the fragrance spreads everywhere. We have molecules bouncing around randomly, gradually dispersing.
This happens everywhere: in the atmosphere, in your coffee, inside your own cells. It’s how oxygen spreads into your bloodstream, how heat equalizes in a room, and how ink spreads in water without you stirring it. Unlike advection, which relies on bulk movement (like wind or ocean currents), diffusion is a slow, passive process driven by molecular motion.
Diffusion works at every scale, from gas clouds in space to chemical reactions inside a single cell. It’s the universe’s way of making sure nothing stays too concentrated in one spot for too long.