In optics, reflectivity is the reflectance (the ratio of reflected power to incident power, generally expressed in decibels or percentage) at the surface of a material so thick that the reflectance does not change with increasing thickness, i.e., the intrinsic reflectance of the surface, irrespective of other parameters such as the reflectance of the rear surface. The concept is of some importance in telecommunication.

Note: The term reflectivity is no longer in common use, but reflectance is.

Source: from Federal Standard 1037C