A exorcism is a religious rite to cast out a demon or evil spirit from a person, object or place.
Cheche, Kokumo and Regan MacNeil were all victims of possession by the demon Pazuzu, Father Lankester Merrin was the one that cast out Pazuzu from Kokumo and Cheche and Father Damien Karras was the priest who exorcized Regan. In The Exorcist III, Karras`s corpse was possessed by The Gemini Killer and was killed by William F. Kinderman, to cast the killer`s spirit away from the body.