What is the circular pavement at the end of a runway called? What is the circular pavement at the end of a runway called where aircraft turn 180 degrees to then backtrack? Is the term runway turning bay appropriate for that? 

The problem is that there is a special NOTAM at our aerodrome for big aicraft (more than 75 tons of weight) to turn not on the runway but only in this adjacent part, and with maximum radius due to fragile asphault pavement.
It is called the turn pad. This area(pad) is usually located at the thresholds of the runway, where no taxiway exists, so in order for the pilots to make a full-length takeoff, (or taxi to the Apron) they have to backtrack amd make a U-turn on there. Gernerally speaking, these pads exist on small airports. You can see the turn pads at the 2 thresholds of the runway. Skiathos Airport, Greece
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