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Angle subtended
An angle formed when two lines from a point meet two points on a circle. For example, chord $AC$ subtends an angle at the center $\angle AOC$ and an angle at the circumference $\angle ABC$.
Arc
A part of the circumference of a circle between two points.
Center
The point inside a circle that is the same distance from every point on the circumference.
Chord
A line segment joining two points on the circumference of a circle.
Circumference
The curved boundary (perimeter) of a circle.
Cyclic quadrilateral
A quadrilateral whose vertices all lie on the circumference of the same circle.
Diameter
A chord that passes through the center of the circle, its length is twice the radius.
Radius
A line segment from the center of a circle to a point on the circumference.
Secant (in the circle)
A line that intersects a circle at two points.
Segment
The region bounded by a chord and the arc between the chord’s endpoints.
Tangent (to a circle )
A straight line that touches the circle's circumference at exactly one point, called the point of tangency, without crossing into the interior