Polar moment of inertia
The polar second moment of area (also referred to as "polar moment of inertia") is a measure of an object's ability to resist torsion as a function of its shape.  It is one aspect of the second moment of area linked through the perpendicular axis theorem where the planar second moment of area uses a beam's cross-sectional shape to describe its resistance to deformation (bending) when subjected to a force applied in a plane parallel to its neutral axis, the polar second moment of area uses a beam's cross-sectional shape to describe its resistance to deformation (torsion) when a moment (torque) is applied in a plane perpendicular to the beam's neutral axis.  While the planar second moment of area is most often denoted by the letter, , the polar second moment of area is most often denoted by either, , or the letter, , in engineering textbooks.