What is the meaning of exorcism which is used to treat abnormal behavior since ancient times?

Removing evil forces from an individual through countermagic and prayer

One ancient theory that is still encountered today holds that abnormal behaviour can be explained by the operation of supernatural and magical forces such as evil spirits (bhoot-pret), or the devil (shaitan). Exorcism, i.e. removing the evil that resides in the individual through countermagic and prayer, is still commonly used. In many societies, the shaman, or medicine man (ojha) is a person who is believed to have contact with supernatural forces and is the medium through which spirits communicate with human beings. Through the shaman, an afflicted person can learn which spirits are responsible for her/his problems and what needs to be done to appease them