a particular nonparametric estimator of a function

a piecewise cubic polynomial such that the function, its derivative and its second derivative are continuous at the interpolation nodes

a piecewise cubic polynomial with the function value, first derivative, and second derivative being continuous at each interior knot

A curve fitted through, or close to, all the points on a graph. It takes points in small groups, and fits a simple cubic equation to fit these, then moves on to the next group. q.v. bestfit.