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.

Surface reflectance may be subdivided into diffuse or Lambertian reflectance and specular, or Phong reflectance. The apparent reflectance for an ideal Lambertian surface is independent of the observer. This means, for a matt surface, the object brighness will be the same regardless of the angle between the observer and the surface. This contrasts with a shiny surface, where the apparent brightness is highest when the observing angle is equal to the source angle. Most real objects have some mixture of Lambertian and Phong qualities.

Source: from Federal Standard 1037C

Note: In climatology, reflectivity is called albedo.