A demon (or daemon, from Ancient Greek, δαίμων daímōn), is a supernatural being from various religions, occultisms, literatures, and folklores that is described as something that is not human and, in ordinary (almost universal) usage, malevolent. The original neutral Greek word "daimon" does not carry the negative connotation initially understood by implementation of the Koine (Hellenistic and New Testament Greek) δαιμόνιον (daimonion), and later ascribed to any cognate words sharing the root, originally intended to denote a spirit or spiritual being.

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" which may cause demonic possession, to be addressed with an act of exorcism. In Western occultism and Renaissance magic, which grew out of an amalgamation of Greco-Roman magic, Jewish demonology and Christian tradition, a demon is a spiritual entity that may be conjured and controlled. Many of the demons in literature were once fallen angels.^{[citation needed]}