Acronym form: ADD.
A much-debated psychological disorder characterized by a constant need to switch activities and/or hyperactivity.
Contrary to popular belief, ADD is a real disorder, and there are people who actually benefit from ritalin. Unfortunately, a lot of normal people with short attention spans claim to have ADD in order to attract sympathy. A good way to differentiate someone with ADD: people with ADD can't sit still for long even when they're doing something they like, they constantly fidget with their hands (I mean CONSTANTLY), and they don't go around telling everyone they meet that they have ADD.
Person with short attention span: "Hey, show me some respect! I have ADD, you know!"
Person with ADD: "Sorry, I'm getting kind of bored already. I guess I just have a short attention span."