Circles
A circle is a set of points in a plane that are equidistant from a fixed point. A circle is denoted by 
.
The fixed point is called the center and the distance from the fixed point to the set of points is the radius.
A segment from the center to the point on the circle is also called the radius.
A segment that joins two points on a circle is called a chord.
A chord that passes through the center is called a diameter.
A secant is a line that contains a chord.
Diameter of a circle is twice the radius.
Congruent circles are circles that have congruent radii.
Concentric circles are circles that have the same center, and lie on the same plane.
A sphere is a set of points that are equidistant from a fixed point.
The terms used for circles are also used for spheres.
If a circle is drawn around a polygon, where the vertices of the polygon is touching the circle, it is said that the circle is circumscribed about the polygon.
If a polygon is drawn inside a circle where the vertices of the polygon is touching the circle then it is said that the polygon is inscribed in a circle.
