A demon is an evil spirit. Demons are found in many religions; in Christianity they are considered to be fallen angels, angels who rebelled against God. In the Western monotheistic faiths (Judaism, Christianity, and Islam) the chief of demons is Satan (though in Islam he is called Iblis).
Many cultures have developed a rich mythology of demons. The study of the different types of demons is called demonology, while the worship of demons is demonolatry. Specific demons or types of demons include Satan, Lilith, incubi and succubi.