1. a keen mind, formidable intellect
2. that which an intelligent person uses to predict future events instead of a "crystal ball"
This term was coined by comedian Adam Carolla on his morning radio show to combat a harpy lawyer who was insisting that she couldn't possibly predict what an obvious outcome would be because she didn't have a crystal ball.
Douchebag: You don't know that. You don't have a crystal ball.
Reasonable Person: I don't need a crystal ball; I have a crystal brain.