BISECTOR, a point which divides a line segment into two equal parts (also used in the Latin feminine form "bisectrix") ; a line which divides a plane angle into two equal angles; a plane that divides a dihedral angle in the same way; or, more generally speaking, any line or plane that divides any geometric magnitude (like a parallelogram, sphere, or cube) into two congruent parts. In crystallography, it denotes the bisector of the angle between the optic axes.