Degeneracy (mathematics)

In mathematics, a degenerate case is a limiting case in which a class of object changes its nature so as to belong to another, usually simpler, class.

A point is a degenerate case of a circle, namely one with radius 0. The circle is a degenerate form of an ellipse, namely one with eccentricity 0.
The line is a degenerate form of a parabola if the parabola resides on a tangent plane.
A hyperbola can degenerate into two lines crossing at a point, through a family of hyperbolas having those lines as common asymptotes.
See "General position" for other examples.