The Collaborative International Dictionary of English v.0.48:

Membrane \Mem"brane\, n. [F., fr. L. membrana the skin that
   covers the separate members of the body, fr. L. membrum. See
   Member.] (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.
   [1913 Webster]

   Note: The term is also often applied to the thin, expanded
         parts, of various texture, both in animals and
         vegetables.
         [1913 Webster]

   Adventitious membrane, a membrane connecting parts not
      usually connected, or of a different texture from the
      ordinary connection; as, the membrane of a cicatrix.

   Jacob's membrane. See under Retina.

   Mucous membranes (Anat.), the membranes lining passages and
      cavities which communicate with the exterior, as well as
      ducts and receptacles of secretion, and habitually
      secreting mucus.

   Schneiderian membrane. (Anat.) See Schneiderian.

   Serous membranes (Anat.), the membranes, like the
      peritoneum and pleura, which line, or lie in, cavities
      having no obvious outlet, and secrete a serous fluid.
      [1913 Webster]

WordNet (r) 3.0 (2006):

membrane
    n 1: a thin pliable sheet of material
    2: a pliable sheet of tissue that covers or lines or connects
       the organs or cells of animals or plants [syn: membrane,
       tissue layer]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "membrane":
   allantoic membrane, amnion, amniotic sac, arachnoid membrane,
   basement membrane, coat, coating, collop, conjuctiva, covering,
   cut, deal, disk, eardrum, feuille, film, flap, foil, fold, hymen,
   lamella, lamina, laminated glass, laminated wood, lap, leaf,
   maidenhead, membrana serosa, membrana tympana, meninges, meninx,
   neurilemma, pane, panel, patina, peel, pellicle, pericardium,
   perineurium, plait, plank, plate, plating, pleura, ply, plywood,
   rasher, safety glass, scum, serosa, sheet, skin, slab, slat, slice,
   table, tablet, tympanic membrane, tympanum, velum, veneer, wafer