Arc  -  Two endpoints on a circle and all of the points on the circle between those two endpoints.

Center  -  The point from which all points on a circle are equidistant.

Central Angle  -  An angle whose vertex is the center of a circle.

Chord  -  A segment whose endpoints are on a circle.

Circle  -  A geometric figure composed of points that are equidistant from a given point.

Circle Segment  -  The region within a circle bounded by a chord of that circle and the minor arc whose endpoints are the same as those of the chord.

Circumscribed Polygon  -  A polygon whose segments are tangent to a circle.

Concentric Circles  -  Circles that share a center.

Diameter  -  A chord that intersects with the center of a circle.

Equidistant  -  The same distance. Objects can be equidistant from one another.

Inscribed Polygon  -  A polygon whose vertices intersect with a circle.

Major Arc  -  An arc greater than 180 degrees.

Minor Arc  -  An arc less than 180 degrees.

Point of Tangency  -  The point of intersection between a circle and its tangent line or tangent segment.

Radius  -  A segment with one endpoint at the center of a circle and the other endpoint on the circle.

Secant Line  -  A line that intersects with a circle at two points.

Sectors  -  A region inside a circle bounded by a central angle and the minor arc whose endpoints intersect with the rays that compose the central angle.

Semicircle  -  A 180 degree arc.

Tangent Line  -  A line that intersects with a circle at only one point (the point of tangency).

Tangent Segment  -  A segment with one endpoint at the point of tangency of a circle and the other endpoint on the tangent line.