Where a family of entities can be defined in terms of parameters, and the limiting case produces an entity which is of a different nature. For example, the general quadratic function is in the form y = α‎x² + β‎x + γ‎, and the graph is a parabola. As α‎ decreases the curvature of the parabola steadily decreases, and the limiting case as α‎ → 0 is a straight line, which is a degenerate parabola.