某个截面对于一个轴的极惯性矩，又称截面二次极矩，是对于该界面对于该轴惯性的一种衡量，其定义为：

其中：为微元距轴的距离。

Polar moment of inertia is a quantity used to predict an object's ability to resist torsion, in objects (or segments of objects) with an invariant circular cross section and no significant warping or out-of-plane deformation. It is used to calculate the angular displacement of an object subjected to a torque. It is analogous to the area moment of inertia, which characterizes an object's ability to resist bending and is required to calculate displacement.