any right line passing through the center of a figure or body, as a circle, conic section, sphere, cube, etc., and terminated by the opposite boundaries; a straight line which bisects a system of parallel chords drawn in a curve.-a diametral plane.-the length of a straight line through the center of an object from side to side; width; thickness; as, the diameter of a tree or rock.-the distance through the lower part of the shaft of a column, used as a standard measure for all parts of the order. see module.