Circle, closed plane curve every point of which is at equal distance from a fixed point (center). A line segment between the center and the curve is called a radius (r); the word is also used for the length of that segment. A line segment whose 2 ends lie in the curve is a chord, and a chord passing through the center point is called the diameter (d). The distance around the edge is known as the circumference (C); C = πd. The area of a circle (A) is given by A = πr2. The circle has been used to symbolize eternity, the universe, or heaven.