A circle is a closed curve consisting of all points in a plane at a fixed distance from a central point, called the radius. A circle is simple closed curve and does not intersect itself. The word circle derives from the Greek κίρκος/kirkos, meaning ring or circle. Circles are conic sections: if two intersecting lines are drawn so that the point of intersection lies on the circumference of each line, then their intersection is a circle. If two parallel lines are drawn so that one line lies on the circumference of the other, then their intersection is also a circle. Every straight line through the center of a circle perpendicular to its plane bisects it into two equal semicircles; every diameter of every such semicircle divides it into two equal quadrants.
The parts of a circle can be divided into three groups: those on the circumference (the perimeter), those inside the interior but not on the circumference, and those in between them, which are called chords. The following nine terms are used when discussing circles:
* Center: The point within a circle or sphere that is equidistant from all points on the edge or surface
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