Traditionally an Exorcist is a hero or heroine who actively combats otherworldly evils trying to gain an anchor in the physical world. Exorcists specifically combat possession, whether it is possession of a person, object or location. Exorcists try to defeat/expel possessors without destroying the host. While "Exorcist" is a formal religious ceremonial title, it may be used by just about anyone in fiction who actively combats possessions.