Zombies are fictional undead creatures that are typically depicted as mindless, reanimated corpses with a hunger for human flesh, and particularly for human brain in some depictions. Intimately tied to the conception of the modern zombie is the "zombie apocalypse"; the breakdown of society as a result of an initial zombie outbreak which spreads.
In a zombie apocalypse, a widespread (usually global) rise of zombies hostile to human life engages in a general assault on civilization. Victims of zombies may become zombies themselves. This causes the outbreak to become an exponentially growing crisis: the spreading "zombie plague/virus" swamps normal military and law enforcement organizations, leading to the panicked collapse of civilian society until only isolated pockets of survivors remain, scavenging for food and supplies in a world reduced to a preindustrial hostile wilderness.