definition circle.

A circle is the set of points equidistant from a point C(h,k) called the
center. The fixed distance r from the center to any point on the circle is
called the radius.
The standard equation of a circle with center C(h,k) and radius r is as follows.
<br /> (x-h){^2} + (y-k){^2} = r{^2}<br />
Reference.
http://www.analyzemath.com/CircleEq/Tutorials.html