Radius: A line joining the centre to any point on the circle. Diameter: Two times the radius is the diameter. Chord: A line segment joining any two points on the circle is chord. Major Arc and Minor Arc: Any two points A and B of a circle divide the circle into two points. If the two parts are unequal, the smaller part is called minor arc and the larger part is called major arc.

A circle divides the plane into three parts, its interiorexterior and the circle itself.
The circle along with its interior is called the circular region.
A chord divides a circular region into two parts called the segments of the region.
The smaller segment is called the minor segment and the larger segment is called the major segment

Theorem 1: In a circle, the perpendicular from the center to the chord bisects the chord.

Theorem 2: Chords of a circle which are equidistant from the centre are equal.

Given: OM = ON (radii of the circle)
To prove: AB = CD
Construction: Join AO and CO
Proof: Consider triangles AMO and CNO
OM = ON
Angle M = Angle N = 90^{o}
Angle AOM = CON (common angle)
Hence triangles AMO and CNO are similar.
Hence AB = CD