Undead is the collective name for all types of supernatural entities that are deceased yet behave as if alive. Undead may be spiritual, such as ghosts, or corporeal, such as animated corpses. Undead are featured in the legends of most cultures and in many works of fiction, especially fantasy and horror fiction.
Corporeal undead have an animated physical body that is otherwise biologically deceased.
Incorporeal undead have no tangible form, but exist in the world of the living as spiritual entities'.
Mary Shelley's novel Frankenstein introduced a new variant of undead, the dead brought back to "life" by science, though Frankenstein's creature bears some similarity to a golem. Similar works include H. P. Lovecraft's short story "Herbert West; Reanimator" and the Re-Animator film inspired by the story.
Both legend and popular culture discuss various methods for creating undead creatures. Most involve the reanimation of a corpse, as with zombies, skeletons, and ghouls. Regarding ghosts, the spirit lives on after death, forming an intangible physical body that often mirrors the one the spirit had in life.
In some cases, the undead, especially skeletons and zombies, are under the control of a sorcerer. In other cases, such as zombies as depicted in film and vampires, the undead existence is passed on like a curse or disease. With liches, the necromancy 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 fiction and folklore, undead creatures are often hostile toward the living. Defending against the undead is often difficult as they are usually depicted as being resistant to normal attacks. Nonetheless, they are often vulnerable to sacred or blessed objects, such as crosses and holy water.
Vampires traditionally can be killed by a stake through the heart or by decapitation, though various traditions have different means of dealing with them. the best way to kill a vampire?
Zombies can often continue to attack when dismembered. To dispatch them, it may be necessary to destroy the head, kill the person who reanimated them, or destroy a significant portion of their body. Firearms, such as shotguns and high-caliber rifles and handguns may be effective against zombies, while low-caliber pistols and rifles may be ineffective.
Incorporeal undead are difficult to defend against because normal physical weapons pass harmlessly through their forms. In many video games and role-playing games, ghosts can only be dispatched by enchanted or silver weapons. 
However, in other fiction the only way to get rid of them permanently is to discover what duty or task they failed to complete in life (an example of this is found in Chapter 4 of The Monk by Matthew Gregory Lewis 
Undead are often depicted as vulnerable to sunlight and fire. They may also perish when their creator is likewise dispatched. Undead may be unable to cross certain symbolic boundaries or even natural barriers like running water.
In some cultures, various plants are said to repel the undead. Examples include garlic and wolfsbane, as well as rosewood, rowan, hazel, willow, and holly. This modern tradition appears to be based on pre-Christian belief that some plants are sacred. Additionally, a line of salt is sometimes said to act as a barrier to the undead.
Games and popular culture
Undead are a popular adversary in fantasy and horror settings. They feature prominently in many role-playing games, computer role-playing games, MMORPGs and strategy games. In such games, special rules are often given for the undead.
Warcraft III provides an entire races of Undead warriors.
Jacques Derrida used the myth of the undead as a means to deconstruct the binary opposition between life and death.