The Illuminati are the secret society that rule the world. Originally the Bavarian Illuminati of 1776, the phrases has reoccurred over the years, more popularly used now to describe groups like the Bilderberg group, the idea of the New World Order, made up of the most powerful people, controlling us in an undemocratic fashion. Possibly lizards.
In the Marvel Universe, the Illuminati are a group invented by Brian Bendis that comprise the cleverest, most powerful "good" people in the Marvel world, state leaders, great thinkers, great inspirers. They were originally Mister Fantastic, Iron Man, Professor X, Namor, Black Bolt and Doctor Strange, though their membership has changed over the years, with Medusa, Beast, Black Panther, Captain America and Amadeus Cho, Medusa, Yellowjacket, Captain Britain and "Doc Green" Hulk joining their ranks.
Well, last week Marvel registered the trademark for "Illuminati" for "Comic books; printed periodicals in the field of comic book stories and artwork".
This usually means that Marvel are planning a comic book of the same name and are getting the trademark in early, or that the TV/movie/games wing of the publisher is pursuing something along those lines and they want to shore up the comic book name rights against anyone who fancies trying it on, or someone is proposing a legal challenge.