A selectively permeable membrane, also termed a semipermeable membrane, is a type of polymeric (a large molecular structure made of many repeated smaller pieces) membrane that permits the diffusion of molecules or ions through it by means of diffusion (the mixing of different substances).
Semipermeable membranes can be either biological or synthetic. Many factors influence the rate of diffusion: pressure, concentration, and temperature of the molecules or solutes on either side of the membrane. This process is called osmosis.
An example of a selectively permeable membrane is the lipid bilayer that surrounds cells and allows some substances to pass in and out of the cell.