Phantom (noun)

1. An apparition of a person or thing, especially one that is believed to be supernatural.
2. Something that exists in perception but has no physical reality.

Origin:

Middle english fantome, from anglo-french, from latin phantasma, from greek phantasma, from phantazein to make visible, from phantos visible, from phainesthai to appear.

Examples:

1. The ghost was a phantom that haunted the house.
2. He saw a phantom of his dead wife in his dreams.
3. The phantom pain was a side effect of the amputation.
4. The phantom limb phenomenon is well known among amputees.
5. The phantom car was an optical illusion caused by the fog.