A wave that is guided along the interface between two different media or by a refractive index gradient. Note 1: The field components of the wave diminish with distance from the interface. Note 2: Optical energy is not converted from the surface wave field to another form of energy and the wave does not have a component directed normal to the interface surface. Note 3: In optical fiber transmission, evanescent waves are surface waves. Note 4: In radio transmission, ground waves are surface waves that propagate close to the surface of the Earth, the Earth having one refractive index and the atmosphere another, thus constituting an interface surface.