interface
/ IN-ter-face /
Look at a glass of soda. There is a clear line where the liquid stops and the air begins; bubbles cling to the glass at the line where liquid meets solid; and the fizz rises through the surface where liquid meets gas. Each of these dividing lines is an interface — the thin region where two different materials meet and neither fully belongs.
More precisely, an interface is the boundary between two distinct phases — such as solid and liquid, liquid and gas, or two liquids that will not mix. It is not a sharp mathematical plane but a thin transition zone, often only a few molecules thick, where the arrangement and energy of molecules differ from either bulk phase on its sides. Because the molecules there are pulled unequally by their two different neighbours, an interface always carries extra energy, expressed as surface or interfacial tension.
Interfaces matter because an astonishing amount of chemistry, biology, and technology happens precisely at them: a cell's membrane, a catalyst's surface, the wall of a soap bubble, a paint film drying, an electrode in a battery. The caveat is one of language: when one of the phases is a gas or a vacuum, people usually say "surface"; "interface" is the general word that covers every two-phase boundary, including those buried out of sight.
Pour oil onto water and the two refuse to blend, leaving a sharp horizontal interface between them. That single thin boundary is where an emulsifier must do all its work to mix them.
An interface is the thin region where two phases meet.
Rough rule of thumb: "surface" = boundary with gas or vacuum; "interface" = boundary between any two condensed phases or, more broadly, any two phases at all. The energy cost of these boundaries is why drops are round and why finely divided matter is so reactive.