Diffusion is the net movement of anything, such as atoms, ions, molecules, or energy, from a region of higher concentration to a region of lower concentration. It is driven by a gradient in Gibbs free energy or chemical potential. Diffusion is a natural and physical process that occurs in many fields, including physics, chemistry, biology, sociology, economics, statistics, data science, and finance. The concept of diffusion is common to all of these fields, and it involves a substance or collection undergoing diffusion spreading out from a point or location at which there is a higher concentration of that substance or collection.

In biology, diffusion is the process of movement of molecules under a concentration gradient. It is an important process occurring in all living beings, and it helps in the movement of substances in and out of the cells. The molecules move from a region of higher concentration to a region of lower concentration until the concentration becomes equal throughout. Liquid and gases undergo diffusion as the molecules are able to move randomly. Diffusion can be classified into two main types: simple diffusion and facilitated diffusion. Simple diffusion is a process in which the substance moves through a semipermeable membrane or in a solution without any help from transport proteins. Facilitated diffusion refers to a type of passive diffusion where channels are lined by proteins that facilitate the diffusion of larger molecules through a semipermeable membrane.

In physics and chemistry, diffusion is the movement of particles from an area of higher concentration to an area of lower concentration as driven by thermal energy. The particles suspended in liquids and gases, for instance, collide with each other, resulting in their random constant motion. This motion due to the collisions of particles is called pedesis or the Brownian movement. The rate of flow of the diffusing substance is found to be proportional to the concentration gradient. The diffusivity is the constant of proportionality and governs the rate of diffusion.