an illusion
a ghost or spectre
a figment of the imagination; a fantasy
phantasmal /fanʹtazm(ə)l/ adj
phantasmic /fanʹtazmik/ adj
Middle English fantasme from Old French fantasme from Latin phantasma, from Greek, from phantazein to present to the mind, from phantos visible. The spelling of this and many other words now beginning with ph- changed from the 16th cent. onwards under the influence of Latin and Greek