The abrupt change in direction of a wave front at an interface between two dissimilar media so that the wave front returns into the medium from which it originated. Note 1: Reflection may be specular (i.e., mirror-like) or diffuse (i.e., not retaining the image, only the energy) according to the nature of the interface. Note 2: Depending on the nature of the interface, i.e., dielectric-conductor or dielectric-dielectric, the phase of the reflected wave may or may not be inverted.