Exorcism is the practice of evicting demons or other evil spiritual entities which are supposed to have possessed (taken control of) a person or object. The practice is quite ancient and still part of the belief system of many religions.
The person performing the exorcism, known as an exorcist, is often a priest, or an individual thought to be graced with special powers or skills. The exorcist may use religious material, such as prayers and set formulas, gestures, symbols, icons, amulets, etc.. The exorcist often invokes some benign supernatural power to actually perform the task.
In general, possessed persons are not regarded as evil in themselves, nor wholly responsible for their actions. Therefore, exorcism is generally thought more as a cure than as a punishment. However, the two concepts are often confused in practice, and exorcism has often been (ab)used as a pretext for harsh physical punishment, or even sadism.
The concept of possession by evil spirits and the practice of exorcism are very ancient and widespread, and may originate in prehistoric Shamanistic beliefs.
The Christian New Testament includes exorcism among the miracles performed by Jesus. Because of this precedent, possession was part of the belief system of Christianity since its beginning, and exorcism is still a recognized practice of Catholicism, Eastern Orthodox and some Protestant sects.
In recent times, the practice of exorcism has diminished in its importance to most religious groups and its use has decreased. This is due mainly to a greater understanding of psychology and the functioning and structure of the human mind. Many of the cases that in the past which were candidates for exorcism have been found to be the products of mental illness, and are handled as such. More generally, the change in worldview since the Age of Enlightenment, which put increased value on rationalism, materialism, and naturalism, has led to a decrease in the belief of the supernatural.