A demon (from Koine Greek δαιμόνιον daimonion) or daemon (British English) is a supernatural, often malevolent being prevalent in religion, occultism, literature, fiction, mythology and folklore.
The original Greek word daimon does not carry the negative connotation initially understood by implementation of the Koine δαιμόνιον (daimonion), and later ascribed to any cognate words sharing the root.
In Ancient Near Eastern religions as well as in the Abrahamic traditions, including ancient and medieval Christian demonology, a demon is considered an unclean spirit, a fallen angel, or a spirit of unknown type which may cause demonic possession, calling for an exorcism. In Western occultism and Renaissance magic, which grew out of an amalgamation of Greco-Roman magic, Jewish Aggadah and Christian demonology, a demon is believed to be a spiritual entity that may be conjured and controlled.
The Ancient Greek word δαίμων daimōn denotes a spirit or divine power, much like the Latin genius or numen. Daimōn most likely came from the Greek verb daiesthai (to divide, distribute). The Greek conception of a daimōn notably appears in the works of Plato, where it describes the divine inspiration of Socrates. To distinguish the classical Greek concept from its later Christian interpretation, the former is anglicized as either daemon or daimon rather than demon.
The Greek terms do not have any connotations of evil or malevolence. In fact, εὐδαιμονία eudaimonia, (literally good-spiritedness) means happiness. By the early Roman Empire, cult statues were seen, by pagans and their Christian neighbors alike, as inhabited by the numinous presence of the gods: "Like pagans, Christians still sensed and saw the gods and their power, and as something, they had to assume, lay behind it, by an easy traditional shift of opinion they turned these pagan daimones into malevolent 'demons', the troupe of Satan..... Far into the Byzantine period Christians eyed their cities' old pagan statuary as a seat of the demons' presence. It was no longer beautiful, it was infested." The term had first acquired its negative connotations in the Septuagint translation of the Hebrew Bible into Greek, which drew on the mythology of ancient Semitic religions. This was then inherited by the Koine text of the New Testament. The Western medieval and neo-medieval conception of a demon derives seamlessly from the ambient popular culture of Late Antiquity. The Hellenistic "daemon" eventually came to include many Semitic and Near Eastern gods as evaluated by Christianity.
The supposed existence of demons remains an important concept in many modern religions and occultist traditions. Demons are still feared largely due to their alleged power to possess living creatures. In the contemporary Western occultist tradition (perhaps epitomized by the work of Aleister Crowley), a demon (such as Choronzon, the Demon of the Abyss) is a useful metaphor for certain inner psychological processes (inner demons), though some may also regard it as an objectively real phenomenon. Some scholars believe that large portions of the demonology (see Asmodai) of Judaism, a key influence on Christianity and Islam, originated from a later form of Zoroastrianism, and were transferred to Judaism during the Persian era.