membrane

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A thin, pliable layer of tissue covering surfaces or separating or connecting regions, structures, or organs of a living organism.
• noun A semipermeable layer that bounds a cell or an organelle, typically consisting of lipids and proteins.
• noun A thin, pliable sheet or layer of natural or synthetic material.
• noun A piece of parchment.
• noun Chemistry A thin sheet of natural or synthetic material that is permeable to substances in solution.

from The Century Dictionary.

• noun A thin pliable expansive structure of the body; an expansion of any soft tissue or part in the form of a sheet or layer, investing or lining some other structure or connecting two or more structures.
• noun In entomology, specifically, the membranous terminal part of a hemielytrum; the membrane of the fore wing of a hemipter. See cut under clavus.
• noun A skin prepared for being written on.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) A thin layer or fold of tissue, usually supported by a fibrous network, serving to cover or line some part or organ, and often secreting or absorbing certain fluids.
• noun a membrane connecting parts not usually connected, or of a different texture from the ordinary connection; as, the membrane of a cicatrix.
• noun See under Retina.
• noun (Anat.) the membranes lining passages and cavities which communicate with the exterior, as well as ducts and receptacles of secretion, and habitually secreting mucus.
• noun (Anat.) See Schneiderian.
• noun (Anat.) , the membranes, like the peritoneum and pleura, which line, or lie in, cavities having no obvious outlet, and secrete a serous fluid.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A flexible enclosing or separating tissue forming a plane or film and separating two environments (usually in a plant or animal).
• noun A mechanical, thin, flat flexible part that can deform or vibrate when excited by an external force.
• noun A flexible or semi-flexible covering or waterproofing whose primary function is to exclude water.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a thin pliable sheet of material
• noun a pliable sheet of tissue that covers or lines or connects the organs or cells of animals or plants