Ghosting
Distinct secondary images of a bright light source formed by reflections bouncing between lens element surfaces.
In depth
Ghosts appear diametrically opposite the light source about the frame's centre, so they move predictably as the camera pans — which is how you can compose them away. They are worst in complex zooms with many air-glass surfaces and in lenses shot behind a cheap protective filter, which adds two more highly reflective surfaces right at the front.
Examples in practice
Removing a cheap UV filter eliminates a bright green ghost that had been appearing opposite every street lamp.