Diffusion
Definition
The gradual spreading of particles from crowded areas to less crowded ones, like perfume filling a room.
Technical definition
Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration due to random molecular motion. Lighter particles and higher temperatures speed it up.
Example
A drop of food coloring slowly spreads through still water without any stirring: diffusion evening out the concentration.
Common confusion
Diffusion is not effusion. Diffusion is mixing through another substance, while effusion is gas escaping through a tiny hole into a vacuum.