The diameter of an object is the longest distance between two points on the object.

Line segment that joins two points on a circle and that passes through the centre of the circle.

Therefore, the diameter of a sphere is a line segment that passes through the centre of the sphere and that joins two points on the sphere.

In the case of a regular polygon, the diameter is the diameter of the circumscribed circle of the polygon. If the polygon has an even number of sides, the diameter joins two opposite vertices and passes through the centre of the polygon.

### Properties

Therefore, the diameter is the longest chord of a circle or a sphere.

The term also refers to the measure of this chord.

### Examples

- The radius of a circle is half the length of the diameter.
- The diameter of a square is the length of its diagonal :

### Etymological note

The word “diameter”, derived from the Greek *δια-μετρητοσ,* means “measure”.