From Encyclopedia of Mathematics - Reading time: 1 min
on a rectifiable curve
A parameter for a curve with parametric representation such that the arc length on the curve between two points and is equal to . The parametrization of a curve by the natural parameter is known as its natural parametrization. The natural parametrization of a -times differentiable (analytic) curve with no singular points is also times differentiable (analytic).
See also (the references to) Natural equation.