phantom

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Something apparently seen, heard, or sensed, but having no physical reality; a ghost or apparition.
• noun An image that appears only in the mind; an illusion.
• noun Medicine A model of a human body or body part.
• adjective Resembling, characteristic of, or being a phantom.
• adjective Fictitious or nonexistent, often when intended to deceive.
• adjective Believed to be real even though illusory.
• adjective Being a phantom limb.

from The Century Dictionary.

• noun Appearance merely; illusion; unreality; fancy; delusion; deception; deceit.
• noun A phantasm; a specter or apparition; an imagined vision; an optical illusion.
• noun Same as manikin, 2.
• Apparent merely; illusive; spectral; ghostly: as, a phantom ship.
• noun A phantom crystal.
• noun A map or diagram of the magnetic field made by strewing iron filings upon a plate of glass-or other smooth surface and allowing them to arrange themselves along the lines of force.

from the GNU version of the Collaborative International Dictionary of English.

• noun That which has only an apparent existence; an apparition; a specter; a phantasm; a sprite; an airy spirit; an ideal image.
• noun See Flying Dutchman, under Flying.
• noun (Med.) a swelling, especially of the abdomen, due to muscular spasm, accumulation of flatus, etc., simulating an actual tumor in appearance, but disappearing upon the administration of an anæsthetic.
• adjective Being, or of the nature of, a phantom.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Something apparently seen, heard, or sensed, but having no physical reality; a ghost or apparition; something elusive or delusive.
• noun An image that appears only in the mind; an illusion.
• adjective illusive
• adjective fictitious or nonexistent

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a ghostly appearing figure
• noun something existing in perception only
• adjective something apparently sensed but having no physical reality