Amorphous

"Lacking defined form or shape."

- Ghostly World Dictionary

An amorphous being is something that lacks physical substance.

Definition
Ghosts lack form or shape in most cases, often appearing as mists, vortexes, or brief apparitions that do not appear for long. Being amorphous means they are not physical beings, rather, they are more like energy.\

Synonyms

 * Insubstantiality