According to Wikipedia.org, a zombie (from Haitian Creole: zonbi) can be either a fictional undead monster or a person in an entranced state believed to be controlled by a wizard. These latter are the original zombies, occurring in the West African Vodun religion and its American offshoots Haitian Vodou and New Orleans Voodoo.
Zombies became a popular device in modern horror fiction, largely because of the success of George A. Romero’s 1968 film Night of the Living Dead and they have appeared as plot devices in various books, films and in television shows.
Zombies from George Romero's 1968 movie Night of the Living Dead
Zombie fiction is now a sizeable sub-genre of horror, usually describing a breakdown of civilization occurring when most of the population become flesh-eating zombies – a zombie apocalypse. The monsters are usually hungry for human flesh, often specifically brains. Sometimes they are victims of a fictional pandemic illness causing the dead to reanimate or the living to behave this way, but often no cause is given in the story.