phantom

Phantom is a ghost or something that seems to appear but that doesn't actually exist. (noun)

A ghost that haunts your house is an example of a phantom.

A phantom is something that seems real but does not exist. (adjective)

An example of a phantom is a phantom pain in an amputated leg.

See phantom in Webster's New World College Dictionary

noun

something that seems to appear to the sight but has no physical existence; apparition; vision; specter
something feared or dreaded
something that exists only in the mind; illusion
a person or thing that is something in appearance but not in fact: a phantom of a leader
any mental image or representation: the phantoms of things past

adjective

of, like, or constituting a phantom; not really existing; illusory

See phantom in American Heritage Dictionary 4

also fan·tom (fănˈtəm)

noun

a. Something apparently seen, heard, or sensed, but having no physical reality; a ghost or an apparition.
b. Something elusive or delusive.
An image that appears only in the mind; an illusion.
Something dreaded or despised.

adjective