diffusionnoun [ U ]
/dɪˈfju·ʒən/science (of gases and liquids) the process of spreading into a surrounding substance
biology Diffusion is also the method by which substances pass in and out of cells through their membrane (= outside covering).
physics Diffusion is also the way light spreads when it is reflected from a rough surface or passed through a translucent substance (= substance that allows only some light to pass through).