Diameter
In geometry, the diameter of a circle is a line from one side directly to the opposite side through the center. It can also be defined as the longest chord of a circle. The same explanations can be also used to describe the diameter in a sphere.
The letter d or the symbol ⌀ are often used as a symbol for the diameter.[1] The diameter's length is often called the diameter itself.
The diameter's length is twice the radius,[2] and therefore the relationship between the diameter's length and radius can be shown with the equations:[3]
and