In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for the diameter of a sphere. The word "diameter" is derived from Ancient Greek: διάμετρος (diametros), "diameter of a circle," from διά (dia), "across, through" and μέτρον (metron), "measure."