1. Energy that travels upward from an energy release and then is reflected downward, such as occurs at the base of the weathering or at the surface. Ghost energy usually joins with the down-traveling wavetrain to change the effective waveshape. Sometimes called secondary reflection (which is also applied to other multiples).
2. Energy reflected from the water surface before being picked up by a submerged receiver.
3. A correlation ghost (q.v.) that results from harmonic distortion when using vibroseis.
4. A reflection of light from the front side of a mirror as well as from the silvered back side, thus producing a double image.