A member of a particular Satanic world-wide secret society that reaches into the highest echelons of world governments, and is bent on world domination.
Known to worship the Evil goddess Eris who is known as Discord to the Romans.
Robert Anton Wilson reveals all their secrets in The Illuminatus! Trilogy.
Their sacred text is a book called Principia Discordia which is written in code to convince people that it's just a joke.
Fundamentally, "Discordianism" is a tedious, moronic joke that's been repeated so often it makes Douglas Adams seem fresh in comparison. It's hard to imagine anybody dumb enough still to be amused by it, but some are. Imagine the most oppressively boring in-joke in history being shouted repeatedly into a human ear, forever. That's "Discordianism".