| A | B |
|---|
| Adjacent Arcs | Two arcs that have exactly one point in common |
| Arc | An unbroken part of a circle |
| Arc Length | The string length of an arc |
Arc of a Chord,  | An arc that shares endpoints with a chord,  |
| Central Angle | An angle whose endpoints are on a circle and vertex is at the center |
| Chord | A segments whose endpoints are on a circle |
| Circle | A set of points that are equidistant from a given point |
| Circumference | The distance around a circle |
| Circumscribed Polygon | A polygon with each side a tangent to a circle |
| Common External Tangent | A common tangent that does not intersect the segment whose endpoints are centers of two circles |
| Common Internal Tangent | A common tangent that does intersect the segment whose endpoint are the centers of two circles |
| Common Tangent | A line that is tangent to two circles in the same plane |
| Concentric Circles | Circles that have the same center, but different radii |
| Diameter | A chord that contains the center of a circle |
| External Secant Segment | The part of a secant segment that is exterior to a circle |
| Secant Segment | A segment from a point outside a circle to a point on a circle containing a chord of the circle |
| Inscribed Angle | An angle with the vertex on a circle containing two chords |
| Inscribed Polygon | A polygon with all vertices on a circle |
| Major Arc | An arc formed by a central angle that is > 180 |
| Minor Arc | An arc formed by a central angle that is < 180 |
| Pi | The ratio of the circumference of a circle to its diameter |
| Secant | A line that intersects a circle at exactly two points |
| Tangent | A line that intersecst a circle at exactly one point |
| Radius | A segment whose endpoint are the center of a circle and a point on the circle |
