Torsion is a mechanical loading in which a structural element (such as a shaft or beam) is subjected to twisting forces about its longitudinal axis. This twisting force creates shear stress within the material, causing it to deform angularly. Torsion is commonly encountered in applications involving rotating machinery, such as shafts in motors or turbines. The SI (International System of Units) units for torsion,

Where:

• $\tau$ is the shear stress induced by torsion, 
• $T$ is the applied torque,
• $r$ is the radial distance from the center to the point where the shear stress is calculated,
• $J$ is the polar moment of inertia of the cross-sectional area of the shaft.