A diameter is a straight-line segment that passes through the center of a circle or sphere and whose two end points lie on the circumference of the surface. The center is a point of symmetry within a circle or sphere, with any diameter passing through that center denoted as an axis of symmetry. An infinite number of diameters are possible for any given circle or sphere, and are the longest line segments that terminate on the circumference. With one revolution around a circle or sphere equal to 360 degrees, the two end-points of a diameter are 180 degrees away from one another.

Diameter is an appropriate name for the measure across a circle or sphere, as the word is the union of the Greek roots "dia" (across) and "metros" (to measure). One of the earliest uses for the word diameter came from Euclid of Alexandria, a fourth-to-third century b.c...