I agree that it's both broader and more complicated than what I wrote. At least in some states in the U.S., a neuropsychologist can diagnose ADHD, but can't write prescriptions for medications to treat it. Meanwhile, a psychiatrist can write prescriptions for a patient with ADHD, but might only do so after he reads a diagnosis from a neuropsychologist. Your experience may vary.

Also, diagnoses for disability accommodations (e.g., in education) are a whole different and complicated thing.