A closed curve associated with a knot which is displaced along the normal by a small amount. For a knot parameterized as for
along the length of the knot by parameter , the frame associated with is

where is a small parameter, is a unit vector field
normal to the curve at .

Kaul, R. K. "Topological Quantum Field Theories--A Meeting Ground for Physicists and Mathematicians." 15 Jul 1999. http://arxiv.org/abs/hep-th/9907119.