Power of evil, the devil, a term first defined in Judaism, then Christianity. Satan is here represented as a central individualized evil spirit, or generally as the powers of evil. It is also used in Islam, then with the more general meaning.
Satan in front of God. Painting by Corrado Giaquinto. Ca. 1750. Musei Vaticani, Vatican
The concept of Satan goes through several stages of development in early Judaism, partly related to the development of the concept of God. The more it was seen necessary to separate God from evil, the clearer and stronger the definition of Satan would be.
It is often suggested that Jewish speculation over Satan/God was influenced by the dualism of Zoroastrianism, although these two developments may very well have occurred independently. In Zoroastrianism, the predominantly good is that of Ahura Mazda, and the predominantly evil is that of Angra Mainyu or Ahriman.
In Job 1:7 Satan himself tells God that he is one that has wandered the earth. Although this relates to an early understanding of Satan as a spy for God among human beings, Satan would remain a wanderer even after being defined in full opposition to God. Wandering the earth, he could seduce and provoke actions that would separate human beings from God.
For long in Judaism, Satan was defined to only operate within the confines set by God, and remains subordinate to him. Later, he would challenge God, inciting human beings to disobey the will of God. Largely, Satan represents the powers opposite to God, being his adversary. He is represented as the prince of evil spirits. His powers work so that Satan can act directly through human beings, with his demons being able to take possession of humans.
There never was only one form of defining evil in Judaism, other definitions emerged with the many sects. There are many other names used for him too. "Beelzebul" was derived from a the god of Ekron, mentioned in 2 Kings 1, Baalzebub. Other names for Satan are Mastemah, Belial and Angel of Darkness.
The Christian understanding of Satan is largely in correspondence with that of Judaism: Satan being a separate spirit in direct and absolute opposition to God. In Christianity, Satan heads the very forces against which God will fight the ultimate battle, as this is described in the New Testament book Revelations.
In Islam, when evil is represented as an individual force, the most correct term or name to be used is Iblis; technically, "shaytan" is mainly used to describe the quality or force of evil. There are theological treaties explaining shaytans as an own category positioned between jinns and angels. A shaytan can also be seen as an unbelieving jinn.
Shaytans can take the form of human beings, but are unable to change their feet, which remain hooves.
The devil figure is largely a theological idea that belongs to monotheistic and near-monotheistic religious speculation. More ancient religions hold evil to be a force that is constantly part of the world, in the sense that evil can be handled. f.x. by appeasement of certain gods, and that perfection never is to be found even at the top of the hierarchy of gods. This applies to Ancient Egyptian religion as well as Mesopotamian.
As mentioned above, there is an evil force in Zoroastrianism, which may well have been the model for much of Jewish speculation over Satan. Still, the Zoroastrian view is mainly Gnostic, and despite the many similarities many scholars hold dualism as a separate idea from that of an evil devil. Close to this view comes Isma'ilism which also defines evil according to Gnostic ideas.
Yazidism is a religion that has been accused of Satan worship, as their main deity sometimes is called Satan (Shaytan). Deeming them devil worshippers, as often has happened, corresponds not with their own views, but is a concept given to them by both opponents (usually Muslims) and by inaccuracies of many European writers.
In Alevism there is no concept of evil that corresponds to the concept of Satan.
Druze religion interprets evil into complex categories without distinguishing any independent force similar to Satan.
Ahl-e Haqq denies the existence of a devil, rather they believe that evil is part of each and everyone.
In Baha'i there is no independent evil force, rather evil is considered absence of good.