Defined by a set of points equidistant from a given point (the centre) that form the circumference.

The diameter is a chord that passes through the centre of the circle.

The radius is any line from the centre to the circumference and so is half the length of the diameter.

Circle of radius r
Segment of a circle of radius r
Sector of a circle of radius r
Radius of a circle circumscribing a triangle of sides a, b, c
Radius of a circle inscribed in a triangle of sides a, b, c

See also: Annulus, Arc, Central Angle, Circumference, Concentric Circles, Conic Section, Cylinder, Diameter, Disc, Ellipse, Escribed Circle, Great Circle, Hemisphere, Interior of a Circle, Malfatti Circles, pi, Polygon, Radical Axis, Radical Centre, Radius, Small Circle, Sphere, Tangential.

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Subjects: Geometry