diffusion. 1. the state or process of being widely spread. 2. the spontaneous mixing of the molecules or ions of two or more substances resulting from random thermal motion; its rate is proportional to the concentrations of the substances and it increases with the temperature.iffusion. 1. the process of becoming diffused, or widely spread. 2. the spontaneous movement of molecules or other particles in solution, owing to their random thermal motion, to reach a uniform concentration throughout the solvent, a process requiring no addition of energy to the system.