Autistic savant: A person with autism who is exceptionally gifted in a specialized field. That field may, for example, be mathematics. The autistic savant may be able to do rapid, complex mental calculations. Or the field may be music. The autistic savant may be able to perform a entire piece of music after hearing it only once.
The term "autistic savant" has replaced the old term "idiot savant," which is perjorative and imprecise. A savant (from the French savoir, to know) is a sage, a learned person.