Refers to a ghostly or illusory figure, often with a literary or slightly old-fashioned feel. Not an everyday word for 'ghost'.
A villager said he saw an apparition in the forest at night.
その絵には、かすかな幻姿が描かれている。
A faint phantom is depicted in that painting.
Compound of 幻 (げん, 'phantom, illusion') and 姿 (し, 'figure, form'). The exact historical derivation is uncertain, but the word is conventionally associated with ghostly or illusory appearances.