The
turning circle of a
vehicle describes the smallest circle in which it can turn around. Imagine sitting in a
car, turning the
steering wheel all the way over to one side and then slowly
driving round. The wheels will trace out a circle on the ground that is the car's
minimum turning circle. If you drive faster the car will eventually lose
grip and the circle will get
wider.
The turning circle can be measured for any vehicle from a cruise liner to a tank, although it's most often used when talking about road vehicles.