A demon is a malevolent supernatural entity. Historically, belief in demons, or stories about demons, occurs in religion, occultism, literature, fiction, mythology, and folklore; as well as in media such as comics, video games, movies, and television series.

Belief in demons probably goes back to the Paleolithic age, stemming from humanity's fear of the unknown, the strange and the horrific. In ancient Near Eastern religions and in the Abrahamic religions, including early Judaism and ancient-medieval Christian demonology, a demon is considered a harmful spiritual entity which may cause demonic possession, calling for an exorcism. Large portions of Jewish demonology, a key influence on Christianity and Islam, originated from a later form of Zoroastrianism, and was transferred to Judaism during the Persian era.

Demons may or may not also be considered to be devils: minions of the Devil. In many traditions, demons are independent operators, with different demons causing different types of evils (destructive natural phenomena, specific diseases, etc.). In religions featuring a principal Devil (e.g. Satan) locked in an eternal struggle with God, demons are often also thought to be subordinates of the principal Devil. As lesser spirits doing the Devil's work, they have additional duties— causing humans to have sinful thoughts and tempting humans to commit sinful actions.

The original Ancient Greek word daimōn (δαίμων) did not carry negative connotations, as it denotes a spirit or divine power. The Greek conception of a daimōn notably appears in the philosophical works of Plato, where it describes the divine inspiration of Socrates.

In Christianity, morally ambivalent daimōn were replaced by demons, forces of evil only striving for corruption. Such demons are not the Greek intermediary spirits, but hostile entities, already known in Iranian beliefs.

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.

Belief in demons remains an important part of 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, which is Crowley's interpretation of the so-called "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.