(1) The figure formed by two intersecting half planes and the line (called the axis) of intersection on which they terminate. The half planes are called the sides of the dihedral angle. (2) The number expressing the size of the plane angle formed by intersecting the two sides with a plane perpendicular to the axis. The dihedral angle is termed acute, right or obtuse according as the size of the corresponding plane angle is less than, equal to or greater than 90°, respectively.