A chord that passes through the center of a circle is called a diameter. In Fig. (i), the chord AB passes through the center O. Therefore, AB is a diameter of the circle O.

Properties of a diameter: (i) A diameter is the largest chord of a circle; (ii) All diameters of a circle are equal in length (in Fig. (ii) AB = CD); (iii) Half of the diameter is equal to the radius of a circle (in Fig. (ii) OB = AB/2); (iv) The diameter divides a circle into two equal parts, each part being a semi-circle. In Fig. (ii), the diameter AB divides circle O into two semi-circles called semi-circle ACB and semi-circle ADB.