selective permeability

This term is a technical compound noun used almost exclusively in biological and chemical sciences. It describes a precise mechanism of filtration where a barrier acts as a gatekeeper, ensuring that the internal environment of a cell or organ remains stable despite external changes.

Because it refers to a specific physiological property, it is treated as an uncountable concept. It is not used in casual conversation or metaphorical contexts, as its meaning is strictly tied to the physical movement of solutes and solvents across a semi-permeable membrane.