Circles Definitions

Infinite Unity No beginning No end Continuous The perfect shape

Circle A set of all points in a plane at a given distance from a given point in that plane. The given point is the center. We name a circle by its center.

Radius The segment from the center to a point on the circle. All radii of a circle are congruent.

Chord A segment whose endpoints lie on a circle.

Diameter A chord that contains the center of the circle. It is the longest distance across a circle. (The diameter cuts through the center of the circle. This is what makes it the longest distance.)

Secant A line that contains a chord.

Tangent A line in the plane of a circle that intersects the circle in exactly one point. This point is called the “point of tangency”

Central Angle An angle inscribed within a circle that begins at the center of the circle.

Arc Two endpoints on the circle and the unbroken part of a circle between them. Classified into 3 types: 1) Semicircles – one-half a circle. 2) minor arcs – an arc less than a semicircle. 3) major arcs – an arc more than a semicircle.

Sphere A set of all points in space at a distance r from point o (r = radius and o = center). Many terms used with sphere is the same as those used with circles.

Congruent Circles (or Spheres) Circles (or spheres) that have congruent radii.

Concentric Circles Circles that lie in the same plane and have the same center.

Concentric Spheres Spheres that have the same center.

Inscribed in a Circle A figure drawn inside a circle with each vertex of the figure on the circle.

Circumscribed about a figure. A circle drawn on the outside of a figure with each vertex of the figure on the circle.