Diameter
In geometry, the diameter of a circle is a line from one side directly to the opposite side through the centre. It can also be defined as the longest chord of a circle. These are both explanations to describe the diameter in a sphere too.
The letter d or the symbol ⌀ are often used as a symbol for the diameter. The diameter's length is often called the diameter itself.
The diameter's length is twice the radius, and therefore the relationship between the diameter's length and radius can be shown with the equations:
[math]\displaystyle{ d = 2r }[/math] and [math]\displaystyle{ r = \frac{d}{2} }[/math]