 Torsion of a curve

For a unit-speed curve with nonzero curvature, torsion is $\tau =-B^{\prime }\cdot N$, measuring rotation of the osculating plane.

Torsion is a measure of how a curve twists out of the plane formed by its tangent and normal vectors. In mathematical terms, torsion is defined for space curves, which are curves that exist in three-dimensional space.