Diameter

Diameter, in Geometry, a term employed generally in reference to the conic sections. Each of these curves possesses a centre (q.v.), a point which bisects all lines passing through it from one side of the curve to the other. Any such line passing through the centre is called a diameter. In the case of a circle the diameters are all equal. In the ellipse we have pairs of equal diameters; the longest and shortest are impaired and are called the major and minor axes respectively. In the parabola the centre is at an infinite distance and the diameters consequently parallel and infinitely long. In any conic, parallel chords are all bisected by the same diameter.