Savant syndrome, sometimes abbreviated as savantism, is not a recognized medical diagnosis, but researcher Darold Treffert describes it as a rare condition in which persons with developmental disorders (including autism spectrum disorders) have one or more areas of expertise, ability, or brilliance that are in contrast with the individual's overall limitations. Treffert says the condition can be genetic, but can also be acquired
You can refer to someone as an idiot savant if they seem to be less intelligent than normal people but are unusually good at doing one particular thing. an idiot savant, an autistic with a gift for numbers
An intellectually disabled person who exhibits extraordinary ability in a highly specialized area, such as mathematics or music. People of subnormal intelligence or severely limited emotional range who have prodigious intellectual gifts in a specific area. Mathematical, musical, artistic, and mechanical abilities have been among the talents demonstrated by idiot savants. Examples include performing rapid mental calculations of huge sums, playing lengthy compositions from memory after a single hearing, and repairing complex mechanisms without training. About 10% of autistic people are idiot savants; mentally retarded people may also be savants, though the incidence is much lower. See also autism