We let a control point vary while the rest is held fixed. We show that the locus of the moving control point that yields a zero curvature point on the curve is a developable surface, the regression curve of which is the locus that guarantees a cusp on the curve (ACM, Inc.)

So far, we have thought of an elliptic curve as defined by its equation. However, a specific equation is not unique for determining a locus of points. Simple substitutions such as x' = 2x obviously lead to different equations with the same locus of points that satisfy the equation. Substitutions like this amount to a change of coordinate system. (Charles Daney)

This curve is called the locus of the equation. The locus of an equation is a curve containing those points, and only those points, whose coordinates satisfy the equation. (tpub.com)