An Etymological Dictionary of Astronomy and Astrophysics

In an → optical system, the → envelope of all the → reflected or → refracted rays
(by a → mirror or a → lens respectively) which do not come to a common focal point because of geometrical → aberration. This occurs when parallel rays of light fall on a → concave mirror or when a → convex lens
receives parallel light. In the case of → spherical aberration, the caustic surface has an axis of symmetry.

See also: → caustic; → surface.