A passive net movement of a molecule from higher concentration to lower concentration.
Molecules disperse over time in order to form an equilibrium of molecules on both sides of a membrane.
Diffusion is considered a mode of passive transport because it does not require any energy.
Diffusion occurs at different speeds based on the size of the molecules, how much they are moving, and how severe the concentration gradient is.
The difference in the concentration of molecules over a distance.
This image shows the progression of diffusion from a high concentration of molecules on the left side of the membrane (left box) to an equilibrium on both sides of a membrane (right box).
Source: Biology Concepts & Connections Fifth Edition Neil A. Campbell, et all.