From AMS Glossary
A common type of retroreflector, that is, a reflector that returns a ray exactly parallel to the incident ray.
It is formed by intersecting three mutually perpendicular planes, with the center point of the reflector located at the mutual point of intersection, thus providing a target of eight right-angle corners. If the reflector can only be seen from one side, that one side is all that needs to be built. When the look direction has a very limited angle, only one-eighth of the structure is commonly built. The material used depends on the wavelength of radiation to be reflected.