In general, a curved line, made up of segments and defined by control points (for example, CVs). Types of splines include polylines, cardinal splines, B-splines, and non-uniform rational B-splines (NURBS).

Splines were originally developed for shipbuilding. A way to draw a smooth curve through a set of points was needed. The solution was to place metal weights at points and pass a thin wooden beam between the weights. The beam, called a spline, adopts a minimum energy position with respect to the weights, producing a smooth curve. The influence of each weight is maximum at the point of contact, and decreases smoothly away from that point.