Undead is a collective name for all types of Supernatural entities who were once alive in the normal sense, died, and then continued to exist in the world of the living, in forms spiritual (as ghosts) or corporeal (as animated corpses). Undead of different varieties are featured in the legends of most cultures on earth and in many works of Fiction, especially fantasy and Horror fiction.
The term "undead" was invented by Bram Stoker, as the original title for his novel Dracula.
Mary Wollstonecraft Shelley's novel Frankenstein, or the Modern Prometheus introduced a new variant on the dead that walk again, the dead brought back to 'life' by Science, though Frankenstein's creature also bears some similarity to a Golem. Similar works include H.P. Lovecraft's short story Herbert West, Re-Animator and the Re-Animator film franchise that it inspired.
In the film Frankenstein Meets the Wolfman, a sequel to The Wolf Man, the corpse of the Werewolf Larry Talbot is returned to life when the light of the full moon falls on it, making Talbot technically one of the undead for the remainder of the film and its three sequels.
Additionally a vast number of films have been made specifically concerning the undead, usually vampires, zombies, or mummies.
In some cases, especially with skeletons and zombies, the undead can be under control of a practitioner of magic. In other cases, like with zombies as shown in film and with vampires, the undead existence is passed on like a curse or disease. With liches, the powers of undead are sought after by the participant of a magical ritual that turns them from a living being to a lich. Ghosts are said to be kept in their undead state by willpower, either from a keen desire to remain with the living or from a wish to see something completed that they could not do during their lifetime.
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). 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. This is derived from the traditional notion that vampires could be repelled by the cross. Clerics of evil gods can rebuke and control the undead in a similar fashion, by means of Necromancy.
In Warhammer Fantasy Battles the Undead are split in 2 parts. First there are the Vampire Counts armies, second are the Tomb Kings. Both armies have living skeletons in the list, but otherwise are entirely different, having seperate backgrounds and units.
In many games, undead characters appear in many roles, be it a mindless horde of opponents (often zombies or skeletons), a thoughtful, plotting villain (often the case with vampires). Some games feature undead protagonists, such as Mortal Kombat, Blood Omen: Legacy of Kain, and Vampire: The Masquerade.
Zombies are a popular choce of enemies in survival horror games such as the "Resident Evil"/"Biohazard" series (created by a virus) or supenatural survival horror games like "Forbidden Siren" and "Silent Hill" Series'.
Undead (generally referred to as 'ReDeads') are also an enemy in the Legend of Zelda series.
In many games, such as Final Fantasy and D&D, undead can be damaged by using magical effects that heal normal living beings.
In the popular MMORPG World of Warcraft, undead (aka The Forsaken) are a playable race in the horde faction.