Possession is the state of being controlled by a ghost. All ghosts have the ability of possession, but some friendly ghosts (Nicky and Tara Roland) will not use it in most cases. (Human-related)
When a ghost possesses something, they will dissolve into a blur of steam and act as though they are going straight through it.
Ghosts can only be expelled from a victim by a hard hit, killing the victim, or the Funnel Spell, which are the only two solutions that exist - for an object, the only way to expel a ghost is to destroy it.
Ghosts can be expelled from the victim by the victim coming into contact with a surface at high speed; the ghost will be hurt and temporarily stunned, but expelled from the body. Using the Funnel Spell on a possessed victim will expel the ghost from said victim. Another option is to destroy the item/object and kill the possessed person. When the person is killed, the ghost will be forced out of the body, and when the possessed object is destroyed, the ghost will be forced out of the object.
When a human is possessed by a ghost, their irises will turn green, the shade of ectoplasm, and they will behave and walk jerkily until the ghost is expelled.