A secret organization believed to existed by conspiracy theroists. They are portrayed as an antagonistic force who have manipulated many of the events throughout the world to achieve their own ends - particularly world domination.
The logo the Illuminati that mostly used is triangle (mostly green or yellow) with eye on middle of it.
Illuminati also gained meme status after several people made a "conspiracy therory jokes" with almost anything by tried to "decode the hidden message" that will usually ended up at a number 3 or the word "illuminati", and later associated with MLG meme compilation.