phantom

(noun) a ghostly appearing figure. Synonyms: apparition, phantasm, phantasma, fantasm, specter, spectre. Examples: "We were unprepared for the apparition that confronted us."

(noun) something existing in perception only. Synonyms: apparition, phantasm, phantasma, fantasm, shadow. Examples: "A ghostly apparition at midnight."

(adj) something apparently sensed but having no physical reality. Examples: "Seemed to hear faint phantom bells." "The amputee's illusion of a phantom limb."