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Definitions of Parts of Circles
Introduction: A circle is all points equidistant from one point called the center of the circle. Segments drawn near and through the circle create other geometrical objects which we define.

The Lesson:
We show circle O below. A circle is named based on the name of the point which is the center. The segment OA is a radius of the circle.

Definition: A radius is the segment connecting (sometimes referred to as the “distance between”) the center and the circle itself.
In the diagram below, we show circle O with radius OA.

Segments and lines of interest are:
  • diameter: the segment connecting two points of the circle and containing the center
  • chord: any segment with endpoints on the circle
  • tangent: any segment, which if extended in either direction, will only intersect the circle once
  • secant: a segment or line which passes through a circle, intersecting at two points
In the diagram below, we show circle O.

Angles of interest are:

M Ransom

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