Astral Projection was the supernatural occurrence or ability wherein the consciousness of being who's in a state of slumber or comatose, left their body, into an "astral body" where they could travel into the real world or in different planes of existence, throughout time and space.

Astral bodies

Astral bodies were capable of traveling into people's memories, their dream space, the real world where they could interact with the environment or into the "astral plane". 

Astral plane

The astral plane seemed to reside alongside but separately from the physical plane, allowing any astral bodies to witness real world events but remain incapable of interacting with the physical world as well as being noticed by physical elements, though, they were able to possess people's bodies in order to do so.