An equation which specifies a curve in terms of intrinsic properties such as arc length, radius of curvature, and tangential angle instead of with reference to artificial coordinate axes. Intrinsic equations are also called natural equations.

Yates, R. C. "Intrinsic Equations." A Handbook on Curves and Their Properties. Ann Arbor, MI: J. W. Edwards, pp. 123-126, 1952.

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Weisstein, Eric W. "Intrinsic Equation." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/IntrinsicEquation.html