WordNet
n. a membrane (as a cell membrane) that allows some molecule to pass through but not others
Wikipedia
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, passive transport 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 the egg.
Usage examples of "semipermeable membrane".
Daeman had watched Harman burst through the semipermeable membrane and fall to the floor under the unexpected gravity, just as Daeman had done.
Something martian was still seeping through the semipermeable membrane.
The semipermeable membrane of the dome would permit fresh air to enter and allow waste gases to pass out, but would shunt aside anything as thick as a raindrop.