A wraith is a ghost who has left their body without having died, leaving behind their still living body. In this state a wraith can exist simultaneously in the human and ghost worlds. In wraith form, a person's full personality exists in the ghost form while the living body becomes completely apathetic.
A wraith is formed when a person relinquishes their spirit, commonly referred to as "giving up the ghost". The wraith state can be triggered when a living person attempts to use a ghost portal or experiences a state of melancholy. The wraith state can be ended when the ghost reunites with their body.
Apparently, the longer the two halves of a wraith stay separated, the more likely the soul half is to lose their memories and assume themselves as a normal, dead ghost, and the more the body is to move and emote in a manner that seems like a normal human, save for empty eyes and a lack of joy that hints at the hollow husk they really are.