Membrane
From The School of Biomedical Sciences Wiki
A membrane is a thin, pliable layer of tissue covering surfaces or segregating or connecting regions, structures, or organs of an animal or plant. One of the key feautres of some membranes is their permeabiltiy - or the ability of things to pass though them using various mechanisms. An example of such a feature can be seen in transmembrane proteins.
But not always.... see cell membrane