Any situation whereby a number of zombies, usually viral zombies, have come to exist in a given area and are threatening the local population. There are different classifications of outbreak, from small isolated incedents to a worldwide explosion of zombies. To counteract this problem, there are zombie survivalist groups, such as OZORT, that work to allow people to discuss what to do in the event of various stages of zombie outbreak.
There was a small zombie outbreak in Cambodia a few years back, but the local millitary contained the issue before it became a crisis.
The tendency of the zombie horror subgenre to become remarkably popular approximately every twenty years, then fade away. Zombie outbreaks have been noted during the mid-eighties and the late zeroes.
So named because the fad behaves like a zombie horde, first appearing as a controlled instance, then escalating while no-one's looking and finally making the leap to the general populace with mass mainstream multimedia. Eventually the fad has no demographic left to consume, and rots away.
Yesterday Isaac spent the whole carpool ride telling me about his zombie apocalypse survival plan in detail. He's totally buying into the current zombie outbreak.
Zombieland, Plants vs. Zombies, The Walking Dead... I've had enough! I hope this zombie outbreak dies off soon.