a device that reflects light so that the path of the rays is parallel to those of the incident rays
A reflector used in retroreflective sensing to return the emitted light directly back to the sensor. The most efficient type have corner-cube geometry. Reflective tapes use glass beads or smaller, less efficient corner-cubes.
Any instrument used to cause reflected radiation to return along paths parallel to those of their corresponding incident rays. One type, the corner reflector, is an efficient radar target. Trihedral prisms are used to retroreflect light in measuring atmospheric extinction coefficients.