1. The surface from which an object or wave (light, heat, sound, seismic, etc.) is reflected.

2. An interface which gives rise to a contrast in geophysical properties between the media above and below the boundary. See reflection.

3. A component of an ore microscope, used in reflected-light microscopy. There are two types. The glass plate reflector is oriented at 45° to direct the horizontal light source vertically on to the polished surface of the mineral specimen and then to allow the reflected light to pass through it vertically up to the observer. The half-field prism (or mirror system) reflects light downwards through one-half of the aperture of the objective or lens; the light is then reflected back from the mineral specimen through the other half of the objective, passing behind the prism to the observer.