From AMS Glossary
A general term referring to the radiation reflected from, or scattered back through, a given surface in response to radiation incident on the surface with the same wavelength or wavelength range.
Different definitions arise depending on the directionality of the radiation under consideration. Reflectance in terms of the ratio of reflected to incident irradiance is generally termed albedo, which may also be described in terms of the directional-hemispherical reflectance (when the incident irradiance is from a specific direction), or the bihemispherical reflectance (when the incident irradiance is diffuse). Reflectance of radiance from a specified incident direction into a specified reflected direction is termed the bidirectional reflectance, which may also be described by the bidirectional reflection function (BDRF), the bidirectional reflectance distribution function (BRDF), or the bidirectional reflectance factor.