A semipermeable membrane, also termed a selectively permeable membrane (or a differentially or partially permeable membrane), is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by diffusion—or occasionally by more specialized processes of facilitated diffusion, or passive or active transport. The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility, properties, or chemistry. How the membrane is constructed to be selective in its permeability will determine the rate and the permeability. Many natural and synthetic materials thicker than a membrane are also semipermeable. One example of this is the thin film on the inside of an egg.