Some airports have an area, on the threshold, used to back track the aircraft, mainly because of the their wingspan, they look like a half wheel. Is there any designation for this area?
I don't know if it is official, but across Europe, Middle East, Asia, and north Africa I hear using "turnpad", and many ATC in these regions provide directions like "bactrack via turnpad and continue taxi via taxiway A", that means go ahead to turnpad bactrack and vacate runway via taxiway A.
It sounds like you're talking about these:
I'm not sure if the designation is "official", but pilots in the US (and the FAA in various documents) usually refer to such areas as a "Runway Turnaround Area" or simply "Runway Turnaround".
Their purpose is to allow aircraft which have back-taxied on the runway (or landed and rolled past the taxiway turn-offs) to turn around to position for take-off or return to a taxiway turn-off.
Note that the "runway turnaround" is distinct from a "run-up area": The runway turnaround is considered to be part of the runway, so an aircraft occupying that area is occupying the runway. A run-up area is generally a distinct area, usually off a taxiway, where aircraft can perform engine run-ups prior to entering the runway.