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, and there is significant disagreement as to whether any of these specific entities is actually evil.

The English word devil derives via Middle English devel and Old English dēofol and Latin Diábolus, from Late Greek Diabolos, meaning, slanderer, from diaballein, to slander: dia-, across + ballein, to hurl. The term devil can refer to a greater demon in the hierarchy of Hell.

In other languages devil may be derived from the same Proto-Indo-European root word for deva, which roughly translates as "angel". However, a "deva" or "diva" is not a devil.

Raising the devil

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.

Christianity understands the Devil in the context of the Old Testament. Unlike Manichaeism which teaches a coeval dualism, Christians see the devil(also known as bhoot,pishachi in Hindi,india) as a corrupted or fallen angel. He was Lucifer, an angel in authority before the Creation who fell because of pride and because he waged a war against God.

Thomas Aquinas, in his Summa Theologiae, said:

"An angel or any other rational creature considered in his own nature, can sin; and to whatever creature it belongs not to sin, such creature has it as a gift of grace, and not from the condition of nature. The reason of this is, because sinning is nothing else than a deviation from that rectitude which an act ought to have; whether we speak of sin in nature, art, or morals. That act alone, the rule of which is the very virtue of the agent, can never fall short of rectitude. Were the craftsman's hand the rule itself engraving, he could not engrave the wood otherwise than rightly; but if the rightness of engraving be judged by another rule, then the engraving may be right or faulty." (ST I.63.1, italics added)
Commonly-quoted Bible-texts are:

The grave below is all astir to meet you at your coming; it rouses the spirits of the departed to greet you — all those who were leaders in the world; it makes them rise from their thrones — all those who were kings over the nations. They will all respond, they will say to you, "You also have become weak, as we are; you have become like us." All your pomp has been brought down to the grave, along with the noise of your harps; maggots are spread out beneath you and worms cover you. How you have fallen from heaven, O Lucifer, son of the morning! You have been cast down to the earth, you who once laid low the nations! You said in your heart, "I will ascend to heaven; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of the sacred mountain. I will ascend above the tops of the clouds; I will make myself like the Most High." But you are brought down to the grave, to the depths of the pit. Those who see you stare at you, they ponder your fate: "Is this the man who shook the earth and made kingdoms tremble, the man who made the world a desert, who overthrew its cities and would not let his captives go home?" (Isaiah 14:9-17 - this is commonly held to be a dual prophecy about the King of Babylon and Satan)

"And there was war in heaven. Michael and his angels fought against the dragon, and the dragon and his angels fought back. But he was not strong enough, and they lost their place in heaven. The great dragon was hurled down — that ancient serpent called the devil, or Satan, who leads the whole world astray. He was hurled to the earth, and his angels with him." (Revelation 12:7-9)

The epic poem by John Milton, Paradise Lost, has a stylized depiction of the devil that influenced C. S. Lewis (The Screwtape Letters and Space Trilogy), and the J. R. R. Tolkien characters Melkor and Sauron.

In Islam the Devil is referred to as Iblis, also called the Shaitan (a word referring to evil devil-like beings). According to the Qur'an, God (called Allah in Islam) created the Devil out of "smokeless fire", while he created man out of clay. The primary characteristic of the Devil, besides hubris, is that he has no power other than the power to cast evil suggestions into the heart of men.

According to the verses of the Qur’an, the Devil's mission until the Qiyamah or Resurrection Day (yaum-ul-qiyama) is to deceive Adam's children (mankind). After that, he will be put into the fires of Hell along with those whom he has deceived. The Devil is also referred to as one of the Djinns (genies), as they are all created from the smokeless fires. The Qur'an does not depict Shaitan as the enemy of Allah, for Allah is supreme over all his creations and Iblis is just one of his creations. Unlike the Zoroastrian beliefs, all good and bad deeds are from Allah himself and only he can save humanity from the evils of his universe and his creations. Shaitan's single enemy is humanity. He intends to discourage humans from obeying God. Thus, humankind is warned to struggle (jihad) against the mischiefs of the Shaitan and temptations he puts them in. The ones who succeed in this are rewarded with Paradise (jannath ul firdaus), attainable only by righteous conduct.

He was expelled from the grace of Allah when he failed to pay homage to Adam, the father of all mankind. He claimed to be superior to Adam, on the grounds that man was created of earth unlike himself. Even the other angels showed a degree of suspicion when Allah informed them about the creation of man as the regent (caliph) of all things on Earth, but they ultimately prostrated before Adam to show their homage. However, Iblis, adamant in his view that man is a worthless being, never bowed his head before any other than Allah. This caused him to be expelled by Allah, a fact that Iblis blamed on humanity. Initially, the Devil was successful in deceiving Adam, but once his intentions became clear, Adam and Eve repented to Allah and were freed from their misdeeds and forgiven. Allah gave them a strong warning about Iblis and the fires of Hell and asked them and their children (humankind) to stay away from the deceptions of their senses caused by the Devil. (For a more detailed account, see (Iblis or Shaitan.)

In Hebrew, the biblical word ha-satan means adversary or obstacle, or even "the prosecutor" (recognizing that God is viewed as the ultimate Judge).

In the book of Job (Iyov), ha-satan is the title, not the proper name, of an angel submitted to God; he is the divine court's chief prosecutor. In Judaism ha-satan does not make evil, rather points out to God the evil inclinations and actions of humankind. In essence ha-satan has no power unless humans do evil things. After God points out Job's piety, ha-satan asks for permission to test the faith of Job. The righteous man is afflicted with loss of family, property, and later, health, but he still stays faithful to God. At the conclusion of this book God appears as a whirlwind, explaining to all that divine justice is inscrutable with human intellect. In the epilogue Job's possessions are restored and he has a second family to "replace" the one that died.

There is no evidence in Torah, or in the books of the Prophets and other writings, to suggest that God created an evil being. In fact, the Book of Isaiah, Job, Ecclesiastes, and Deuteronomy all have passages which God is credited for creating both the good and the evil of this world.

The Hebrew word for evil used above is usually translated as 'calamity', 'disaster' or 'chaos'.

In contrast to the Christian traditions and Islam, Hinduism does not recognize any central evil force or entity such as the Devil opposing God but does recognize that different beings (e.g., asuras) and entities can perform evil acts and cause suffering in the world.  Prominent asura is Rahu whose characteristics are similar to Devil's.

However, Hindus, and Vaishnavites in particular, believe that Vishnu incarnates to destroy evil when evil has reached its maximum. (see avatar.) Additionally, the problem of evil is mostly explained by the concept of Karma.