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A circle is defined as the collection of all the points on a plane that are at equal distance from a given fixed point on the plane. This fixed point is called centre of the circle and the fixed distance is called the radius.

A line segment joining the centre of a circle with any point on its circumference is called the radius of the circle.

A line that joins two points on the circumference of a circle is called a chord. A chord that passes through the centre of a circle is called the diameter of the circle. A diameter divides a circle into two equal parts, each is called a semicircle. Diameter is the longest chord of a circle. The diameter of a circle is twice the radius.

The part of the circumference of a circle between two given points is called an arc.  

A chord divides a circular area into two parts called segments they are major segment and minor segments. The region between two radii of a circle and any of the arcs between them is called a sector. The diameter of a circle divides it into two equal segments.