D stands for diameter. In order to facilitate writing and counting, some letters are used to simplify, such as "meter" (symbol "m"), "millimeter" (symbol "mm") and "kilogram" (symbol "kg"). Diameter, the distance from the center of a plane figure or a solid (such as a circle, a cone section, a ball or a cube) to two points on the side, is called diameter. A straight line with a diameter is the symmetry axis of a circle.

The two endpoints of the diameter are on a circle, and the center of the circle is the midpoint of the diameter. Diameter divides a circle into two parts with equal area, and the middle line segment is called diameter (each part becomes a semicircle). The line segment connecting two points on the circumference and passing through the center of the circle is called the diameter of the circle, and the line segment connecting two points on the sphere and passing through the center of the sphere is called the diameter of the sphere.

Properties of diameter:

1. The length of the diameter of the same circle is twice that of the radius, which can be expressed as d=2r or r=d/2.

2. The longest chord in the same circle. It is proved that if AB is the diameter of ⊙O and CD is any string with non-diameter, it can be proved that AB and CD are constants.