Noun (zee day): Derived from the word D-Day, which is a British term used to refer to a variable day on which a combat attack or operation is initiated. Z-Day is the first day of the Zombie Apocalypse and the attack on mankind by zombies, which will lead to a widespread zombie outbreak.

There are a number of zombie survivalist groups worldwide (largely in the US, UK, and Canada), that are already planning what to do when this day comes.

Z-Day will catch mankind completely off guard, and the lack of preparation will lead to countless casualties.

Max Brooks