In physics, an elastic two-dimensional plate is termed a membrane (Latin membrana = parchment) but in chemistry the term denotes a body, usually thin, which serves as a phase separating two other bulk phases. If this body is permeable to the same degree for all components of the adjacent phases and does not affect their mobility, then its only function is to prevent rapid mixing of the two phases. This is then termed a diaphragm. A real membrane must exhibit a certain selectivity, based on different permeability for the components of the two phases, and is then termed a semipermeable membrane. Membranes separating two electrolytes that are not permeable to the same degree for all ions are called electrochemical membranes. It is with these that we are concerned here.

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined.