Brainiac is a fictional character, a DC Comics supervillain and frequent opponent of Superman.
Though at his core Brainiac is formless, most incarnations depict him as a bald, green-skinned alien android from the planet Colu, and one of the most intelligent villains in the DC universe, capable of possessing others, creating and manipulating computer systems, and exerting some control over time and space.
(Thanks Wikipedia for the overview - unsurprisingly, given the nature of geekdom the article about Brainiac goes on for several more pages).
In addition to supervillaindom:
Brainiac is a common modern term used to describe someone exceedingly smart in a certain field. Also a slang term, indicating a highly intelligent person. It can be used in a complimentary manner, or it can be used pejoratively when describing so-called "nerds"
Brainiac is also the password I used when setting up a new identity for a web thing I joined recently. (It gets hard thinking of new passwords all the time and yes, maybe I am a little nerdy, give me a break :)
Unfortunately I didn't quite think it through and now, every time I log into this site, I have to type the password braniac. (Who could still be some kind of supervillain - the name says cereal killer to me)
