I know a lot of people with obsessive qualities, and there's...

I know a lot of people with obsessive qualities, and there's a positive and negative part to it. There's the appeal of someone who's so deadly focused on something that they want or desire - there's a dedication and a single-mindedness that's great. But then if it goes on too long it becomes a psychosis. What's the French term for it? Idee fixe.