Diffusion pertains to the spontaneous net movement of particles down their concentration gradient (i.e. difference in the concentrations of substances or molecules between two areas). In biology, it is a type of passive transport, therefore, it is a net movement of molecules in and out of the cell across the cell membrane along a concentration gradient.
Unlike active transport, diffusion does not involve chemical energy. When molecules move (diffuse) via special transport proteins found within the cell membrane, it is called facilitated diffusion, otherwise it is only simple diffusion. An example of diffusion in biological system is diffusion of oxygen and carbon dioxide across the alveolar-capillary membrane in mammalian lungs.
Diffusion may be a simple diffusion or facilitated diffusion. A simple diffusion is one in which that occurs unassisted. Facilitated diffusion in contrast is an assisted diffusion in a way that it requires a carrier molecule. For instance, polar molecules diffuse across a cell membrane through carrier molecules embedded in the cell membrane.
Word origin: From Latin diffusionem (accusative of diffusio), from verb diffundere