Synonyms for Phantom (same or very similar meaning)
WordNet sense 1 (a ghostly appearing figure):
apparition, phantasm, specter, phantasma, spectre, fantasmWordNet sense 2 (something existing in perception only):
shadowWordNet sense 3 (not actually such; being or seeming fanciful or imaginary):
unrealFrom the ODE community, based on WordNetadd/edit