a: a closed plane curve every point of which is equidistant from a fixed point within the curve b: the plane surface bounded by such a curve
: something (as an anatomical part) in the form of a circle or section of a circle <an arterial circle>—see circle of willis
Geometrical curve, one of the conic sections, consisting of the set of all points the same distance (the radius) from a given point (the centre). A line connecting any two points on a circle is called a chord, and a chord passing through the centre is called a diameter. The distance around a circle (the circumference) equals the length of a diameter multiplied by (seepi). The area of a circle is the square of the radius multiplied by . An arc consists of any part of a circle encompassed by an angle with its vertex at the centre (central angle). Its length is in the same proportion to the circumference as the central angle is to a full revolution.