1. Something apparently seen, heard, or sensed, but having no physical reality; a ghost or an apparition.
2. Something which is elusive or delusive.
3. An image that appears only in the mind; an illusion.
4. Something dreaded or despised.
5. Of, or related to, a nonexistent source of sensation; applied specifically to a bodily limb that has been amputated, but is the apparent source of pain; such as, a phantom limb.
6. A collection of materials used to simulate human tissue in testing radiation absorption and backscattering.
7. A term for a volume of material approximating as closely as possible the density and effective atomic number of living tissue, used in biological experiments involving radiation.