**What is the Diameter of a Circle:** The **diameter **of a circle is the distance across the circle through its center. It is the longest distance of any point on the **circumference **of the circle to any other point on the **circumference**. It is also twice the **length of the radius** of the circle.

For example, if the radius of a circle is 5 units, the **diameter **would be 10 units (since 5 x 2 = 10). The diameter is the line segment that connects two points on the **circumference **of the circle and passes through the center of the circle. It can be represented by the symbol d, where **d = 2r **(r is the radius).