Is it ensure, insure, or assure?
early 15c., "thin layer of skin or tissue," a term in anatomy, from Latin membrana "a skin, membrane; parchment (skin prepared for writing)," from membrum "limb, member of the body" (see member). The etymological sense is "that which covers the members of the body."
membrane mem·brane (měm'brān')
A thin pliable layer of tissue covering surfaces, enveloping a part, lining a cavity, or separating or connecting structures or organs.
A thin sheet of natural or synthetic material that is permeable to substances in solution.