The reflectivity of a surface (e.g. an interface between two optical media) is defined as the ratio of reflected optical power to the incident optical power. Usually, it is applied to cases with specular reflections on exactly flat unstructured surfaces, i.e., not to extended objects and diffuse reflections.
Strictly speaking, according to the rules of radiometry, the term reflectivity should be used only for reflections occurring at one particular pure surface – not for example for reflections at extended objects, such as glass plates or optical resonators, or on rough surfaces; in such cases, the term reflectance is appropriate. However, it is common, for example, to specify reflectivities of laser mirrors or anti-reflection coatings, although these are usually based on dielectric coatings, involving interferences between reflections from many different optical interfaces. One then actually considers a whole thin-film structure, usually having an overall thickness far below 1 mm, as one surface. A more questionable case is a fiber Bragg grating, which can be far more extended.
Relation to Reflection Coefficients
The reflection at an optical surface is also often described with a complex reflection coefficient. Its squared modulus is the reflectivity, and it also carries a complex phase according to the optical phase change upon reflection.