# Definition:Circle/Diameter

## Definition

In the words of Euclid:

A diameter of the circle is any straight line drawn through the center and terminated in both directions by the circumference of the circle, and such a straight line also bisects the center.

In the above diagram, the line $CD$ is a diameter.

## Linguistic Note

There are two adjectival forms of diameter:

### Diametral

Diametral means:

located on or forming a diameter.

### Diametrical

Diametrical means:

relating to or along a diameter.