What Are Diameters?

A circle's diameter stretches across the whole circle

The diameter of a circle is a single line segment that stretches the entire width of the circle, intersecting with the center point and two points around the circumference. The diameter measures how wide the circle is, and is always equal to twice the radius ($2r$).

In the image below, the red segment is the circle's diameter. All the gray segments are also possible diameters (notice how they all pass through the same center point.

Fun fact: the diameter is always a circle's longest chord.