Exorcism is the practice of evicting or destroying spiritual entities. The actual existence of spirits are a matter much religious debate, but in most cultures where such things are believed in, exorcism is also a part of said belief system. The person performing the exorcism is said to be an exorcist.
The exorcist is often a member of the local clergy, thought to be able to assail the spirits with divine help, or an magician of some sort.
For a discussion of the validity of possession, see possession.