1. The passive movement of molecules or particles along a concentration gradient, or from regions of higher to regions of lower concentration.
2. The spontaneous net movement of particles down their concentration gradient (i.e. difference in the concentrations of substances or molecules between two areas).
3. (Cell biology): 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.
Word origin: From Latin diffusionem (accusative of diffusio), from verb diffundere.
Related forms: diffusional (adjective).
Compare: active transport, osmosis.
See also: passive transport.