1. A process that takes place at a solid-liquid interface.

It is made up of ions of one charge type which are fixed to the surface of the solid and an equal number of mobile ions of the opposite charge which are distributed through the neighboring region of the liquid.

2. The area of a charge separation formed when an electrode meets an ionic conductor.

A metal electrode in a water solution forms a specific structure consisting of the metal surface itself, an adjoining layer of adsorbed (adhesion to the surfaces of solids) water molecules and ions, and an outer region of oppositely charged ions diffused in the liquid.

This causes an electric field of considerable intensity.

3. An interfacial region, near the boundary between two different phases of a substance, in which physical properties change significantly.
4. A structure that appears on the surface of a charged object when it is placed into a liquid.

This object might be a solid particle, a gas bubble, a liquid droplet, or a porous body.