n.1. Biology a. A thin, pliable layer of tissue covering surfaces or separating or connecting regions, structures, or organs of a living organism.
b. A semipermeable layer that bounds a cell or an organelle, typically consisting of lipids and proteins.
2. A thin, pliable sheet or layer of natural or synthetic material: the resonating membrane of a kazoo.
3. A piece of parchment.
4. Chemistry A thin sheet of natural or synthetic material that is permeable to substances in solution.