The Devil is the name given to a supernatural entity, who, in most Western religions, is the central embodiment of evil. This entity is commonly referred to by a variety of other names, including Satan, Asmodai, Beelzebub, Lucifer and/or Mephistopheles. In classic demonology, however, each of these alternate names refers to a specific supernatural entity.

Some scholars believe that the notion of a central supernatural embodiment of evil, as well as the notion of angels, first arose in Western monotheism when Judaism came into contact with the Persian religion of Zoroastrianism. Much like classical monotheism, Zoroastrianism has one supreme God, and an evil spirit who chose to be evil, locked in a cosmic struggle where both are more or less evenly matched, though from the beginning Ahura Mazda's triumph is foretold; making Zoroastrianism an ethical dualism. Ahura Mazda ("Wise Lord"), also later known as Ormazd in Middle Persian, is the God of light, or Truth, and Angra Mainyu ("Evil Spirit"), also later known as Ahriman in Middle Persian, is the primeval Spirit of darkness, or the Lie. In a final battle between the forces of good and evil, human souls will be judged in a fiery ordeal of molten metal where the good will pass through as if it were warm milk and those who chose evil will be purified and all will be reunited in the new perfected world. Accordingly, humans are urged to align themselves with Ormazd and his Yazatas ("angels") and to shun His adversary who is the ruler of darkness and his demons, so that they may facilitate the final renovation (Frashō-kereti).

Christianity views Satan as an angel cast from heaven by God, for being prideful, deceitful, and the tempter: all strikingly similar to the story of Ahriman.