Circle, circle line, circumference
A circle is a shape consisting of a curved line completely surrounding an area.
Every part of the line is the same distance from the centre of the area.
Circumference is the distance around a circle.
Circle Terminology
Centre: the point equidistant from all points on the circle.
Circumference: the length of one circuit along the circle, or the distance around the circle.
Diameter: a line segment whose endpoints lie on the circle and that passes through the centre; or the length of such a line segment. This is the largest distance between any two points on the circle. d=2r
Radius: a line segment joining the centre of a circle with any single point on the circle itself; or the length of such a segment, which is half (the length of) a diameter. r = d : 2
Chord: a line segment whose endpoints lie on the circle, thus dividing a circle into two segments.
Secant: an extended chord, a coplanar straight line, intersecting a circle in two points.
Tangent: a coplanar straight line that has one single point in common with a circle («touches the circle at this point»).
Sector: a region bounded by two radii of equal length with a common centre and either of the two possible arcs, determined by this centre and the endpoints of the radii.
Segment: a region bounded by a chord and one of the arcs connecting the chord’s endpoints. The length of the chord imposes a lower boundary on the diameter of possible arcs.
Arc: any connected part of a circle. Specifying two end points of an arc and a centre allows for two arcs that together make up a full circle.
