A ghost ring is a hula-hoop like device used to snare paranormal entities.
Actually it kind of looks like a variation on the peep sight. Not sure if there are different types of ghost ring setups, but my Mossberg 590 front sight is like a huge front pistol sight. The rear sight is like a peep sight, but with a much larger hole (essentially a ring, hence the name). You pretty much put the top of the front sight in the middle of the ring. They are typically found on combat and home defense shotguns.
I'm not sure where the name comes from, but here is my guess:
When properly used (focus on target, top of front sight in middle of ring), the rear ring will be out of focus (because you are focusing on the target, right?). Since the ring is fuzzy in the sight picture, I would guess that it appears that is is there but isn't at the same time. Hence the name Ghost Ring.
I am only speculating on the reason for the name, so if anyone else knows, please post it.
"Ray guns don't vaporize Zorbonians, Zorbonians vaporize Zorbonians" Excerpt from The Far Side