means A thin, flexible sheet of tissue or material that lines, separates, or covers something — letting certain things pass while blocking others.

from From Latin 'membrana,' meaning skin or parchment, especially the thin skin that covers the organs of the body. It comes from 'membrum,' meaning a limb or member — so a membrane was, in a sense, the skin belonging to a body part. The same Latin 'membrana' was also the word for the prepared animal skin used as writing material, which is why the word carries that papery, sheet-like sense to this day.