Origins
The word 'membrane' entered English in the early fifteenth century from French 'membrane,' from Latin 'membrāna.' The Latin word had two primary meanings: the thin skin or tissue covering a body part, and parchment (prepared animal skin used for writing). Both meanings derive from 'membrum' (limb, member, body part), from PIE *mems-ro- (flesh, body part). The original sense of 'membrāna' was literally 'the skin of a limb' — the covering tissue of a membrum — which then generalized to any thin, flexible sheet of tissue.
The PIE root *mems-ro- has interesting reflexes across the family. Sanskrit 'māṃsá' means 'flesh' or 'meat.' Gothic 'mimz' means 'flesh' or 'meat.' The English word 'member' comes from Latin 'membrum' — the same source as 'membrane.' A 'member' is, etymologically, a body part, a limb. To 'dismember' is to cut off the limbs. The 'members' of an organization are its 'body parts' — a metaphor that goes back to the Roman fable of the belly and the members (the body's limbs rebel against the belly, which appears to do no work, until they realize it nourishes them all — a parable of social cooperation told by Menenius Agrippa in 494 BCE).
The dual meaning of 'membrāna' as both tissue and writing material reflects a practical reality. Before the invention of paper (which reached Europe in the twelfth century), the primary writing surfaces were papyrus (processed plant fiber, from Egypt) and parchment (processed animal skin). Parchment — literally a membrane — was the standard writing material of medieval Europe. The finest grades were called 'vellum' (from Old French 'vélin,' from 'veel,' calf), made from the skin of calves, kids, or lambs. Parchment could be scraped clean and reused, producing 'palimpsests' — manuscripts where earlier texts were imperfectly erased and remain partially legible beneath the newer writing.
Development
In modern biology, membranes are fundamental structures at every scale. The cell membrane (plasma membrane) — a phospholipid bilayer studded with proteins — encloses every living cell, controlling the passage of substances in and out. This membrane is approximately 7–8 nanometers thick. Mucous membranes line the internal surfaces of the body — the respiratory tract, digestive tract, and urogenital tract. Serous membranes line body cavities and cover organs (the pleura around the lungs, the peritoneum around the abdominal organs, the pericardium around the heart). The meninges are membranes covering the brain and spinal cord.
In technology, 'membrane' has been adopted for thin, selectively permeable barriers in industrial processes: membrane filtration for water purification, membrane distillation for desalination, and membrane electrodes in fuel cells. The 'membrane keyboard' — a keyboard with keys printed on a flat surface — takes its name from the thin, flexible sheet that responds to pressure.
The adjective 'membranous' describes anything resembling or consisting of a membrane. 'Membranophone' — a musical instrument that produces sound through a vibrating membrane (drums, timpani) — uses the word in its acoustic sense.