From The Art and Popular Culture Encyclopedia
Undead is a collective name for mythological beings that are deceased yet behave as if alive. Undead may be spiritual, such as ghosts, or corporeal, such as Zombies. Undead are featured in the legends of most cultures and in many works of fiction, especially fantasy and horror fiction.
Bram Stoker considered the term "The Un-Dead" for the original title for his novel Dracula, and its use 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.