From Haiti to Hollywood.

Let's start off easy (haha).  What, exactly, is a zombie?

Zom·bie (zŏm'bē)
n.

1. A snake god of voodoo cults in West Africa, Haiti, and the southern United States.
2. Spirit of the dead; a ghost.
3. (a) A supernatural power or spell that according to voodoo belief can enter into and reanimate a corpse. (b) A corpse revived in this way.

The entire concept of zombie-ism comes from Haitian folklore -- or, rather, from the Voodoo culture.  By administering a powder called coup padre, which contained tetrodoxin, malevolent priests could raise the dead from their graves.  In actuality, the toxin created an artificial coma, lowered the subject's pulse and body temperature, and subdued the subject's breathing patterns.  Since there was no way of screening for the toxin, the subject would be mistaken for dead and buried.  The subject would later be exhumed by the priest, and, though physically whole, would have little to no memory.  The subject, working under a comatose trance, would be used as a slave.  So, in the original sense of the word, zombies were only thought of as the living dead, and they certainly didn't exhibit the violent, cannibalistic nature of the modern pop-culture zombie.

So how did we get from Point A to Point B?

It's interesting to note that the early zombie films -- little-known flicks like White Zombie (1932) and Zombies on Broadway (1945) -- followed the Haitian model.  Though some elements changed -- the priest might become a sorcerer or Haiti might become New York City -- the big things stayed the same.  Sometimes the zombies were used for horrific purposes, sometimes for humorous (check out the passing reference in Scared Stiff (1953), wherein the only zombie acts rather like a stoned college student), but the zombies were usually people under hypnosis or a heavy trance, and they were always controlled by a master of some sort.  There were, of course, some exceptions.  Most notably, Things to Come (1936) references a sickness that spreads upon contact, leaving the victim listless, slow, and insensible.

In the late 1950s and early 1960s, zombies became a little more menacing.  Movies like Plague of the Zombies (1965) [note: this is not the first movie to do this, but it's the only one of this era I've actually seen!] created a creature -- typically still under another person's control -- that actually needed to consume human flesh in order to survive as the undead.  Toxins and hypnosis disappeared; suddenly, zombies were being reanimated via supernatural means -- or by science or by aliens -- and they were terrifying creatures.

Enter George Romero.

When he created Night of the Living Dead (1968), Romero drew inspiration from all kinds of sources, including the vampire novel I Am Legend, and in the process he created the modern zombie -- an entirely new monster, by Hollywood standards.  These creatures are obviously undead and they are hell-bent on consuming living flesh, for apparently no reason other than insatiable hunger.  There is no definitive cause for the undead uprising (there is a vague mention of a crashed spacecraft and lingering radiation, but nothing is ever truly determined), and there is certainly no overlord controlling their actions.  The zombies here, much like in Things to Come, spread the plague on contact (in this case, via fluid transfer -- most notably, biting), and conquer their prey with sheer numbers.  They have to come in large numbers: these zombies are frail and slow, and would be easy to pick off individually.  That is, they would be... if they didn't look like our friends, family, and neighbors.

Still, all this only covers the evolution of the zombie, and our initial definition only touches on the traditional voodoo zombie.  This leaves us with an obvious question: How do we define the modern zombie?