membrane

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Biology A thin, pliable layer of tissue covering surfaces or separating or connecting regions, structures, or organs of an animal or a plant.
2. n. Biology Cell membrane.
3. n. A piece of parchment.
4. n. Chemistry A thin sheet of natural or synthetic material that is permeable to substances in solution.

Century Dictionary and Cyclopedia

1. n. 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. The term is used in the widest sense, with little or no reference to the kind of tissue which may be concerned, the membranous quality depending upon thinness and pliability, not upon texture or fabric. No hard parts, as bone and cartilage, come within the definition of membrane. Most membranes are fibrous—that is, consist wholly or in part of some form of connective tissue, in or on which may be other and more special form-elements, as the layers of cells peculiar to the mucous, the serous, and other special membranes. In some cases a sheet of nerve-tissue, or of muscle-tissue, constitutes a membrane, with little admixture of other elements. Some membranes chiefly consist of a network of blood-vessels, with little connective tissue. Most membranes are specified by qualifying terms. See phrases following.
2. n. In entomology, specifically, the membranous terminal part of a hemielytrum; the membrane of the fore wing of a hemipter. See cut under clavus.
3. n. A skin prepared for being written on.

Wiktionary

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

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. n. a thin pliable sheet of material
2. n. a pliable sheet of tissue that covers or lines or connects the organs or cells of animals or plants