An undead is a being in mythology, legend or fiction that is deceased yet behaves as if alive. A common example is a corpse re-animated by supernatural forces by the application of the deceased's own life force or that of another being (such as a demon). Undead may be incorporeal like ghosts, or corporeal like vampires and zombies. The undead are featured in the belief systems of most cultures, and appear in many works of fantasy and horror fiction.
Bram Stoker considered using the title The Un-Dead for his novel Dracula (1897), and use of the term in the novel is mostly responsible for the modern sense of the word. The word does appear in English before Stoker but with the more literal sense of "alive" or "not dead", for which citations can be found in the Oxford English Dictionary. Stoker's use of the term refers only to vampires, and the extension to other types of supernatural beings arose later. Most commonly, it is now taken to refer to supernatural beings which had at one time been alive and continue to display some aspects of life after death, but the usage is highly variable.
The Valley of the Dry Bones, Ezekiel 37:1-14, may be the origin of the undead in literature.
"1. And the hand of YHVH was upon me, and YHVH brought me out by wind and set me down in the valley, and it was filled with bones... 7. ...And there was a voice.. and behold, earthquake, and the bones joined, bone to its bone. 8. And I saw, and behold, sinew upon them, and flesh rose, and skin covered over them, but there was no wind in them... 10. ... And the wind came into them and they lived, and they stood upon their legs, a very great horde."
In Dungeons & Dragons and similar systems, clerics can attempt to "turn" undead by invoking their patron deities or channeling "positive energy" (other-dimensional life energy, which animates and heals living creatures, and is the antithesis of negative energy, which animates and heals undead creatures). This forces the undead creature away from the cleric; powerful clerics are capable of completely destroying weaker undead creatures with this ability. Although the act of turning away the undead relies primarily on power of faith, a holy symbol is usually required as a focus for the divine power being invoked. Clerics of evil gods can rebuke and control the undead in a similar fashion, by instead channeling "negative energy".
In other games, such as Final Fantasy, undead can be damaged by using magical effects that heal normal living beings.