Curvature of a space curve ($\kappa$)

For a unit-speed curve $\gamma$, the unit tangent is $T={\gamma }^{\prime }$ and the curvature is $\kappa =|T^{\prime }|$. It measures the rate at which the tangent direction turns.

For a regular plane curve $(x(u),y(u))$,For a graph $(u,f(u))$, this reduces to $|f^{\prime \prime }(u)|/(1+f^{\prime }(u{)}^2{)}^{3/2}$.

At a regular planar level set of a twice differentiable function, its oriented curvature with normal $\nabla u/|\nabla u|$ is $\kappa =(\Delta u-\nabla u^T(D^{2}u)\nabla u/|\nabla u{|}^2)/|\nabla u|$. Zero curvature at a point gives second-order contact with the tangent line, not a whole straight segment. For example the regular zero level line of $u(x,y)=y+x^3$ has zero curvature at the origin but is curved nearby.

Given a smooth function $\kappa (s)$, setThen $\gamma$ is unit speed and has signed curvature $\kappa$. This gives the existence part of the fundamental theorem of plane curves.