spline

Single knots at 1/3 and 2/3 establish a spline of three cubic polynomials meeting with C parametric continuity. Triple knots at both ends of the interval ensure that the curve interpolates the end points

In mathematics, a spline is a function defined piecewise by polynomials. In interpolating problems, spline interpolation is often preferred to polynomial interpolation because it yields similar results, even when using low degree polynomials, while avoiding Runge's phenomenon for higher degrees.