Outer billiards is a
dynamical system based on a
convex shape in the plane. Classically, this system is defined for the
Euclidean plane but one can also consider the system in the
hyperbolic plane or in other spaces that suitably generalize the plane. Outer billiards differs from a usual
dynamical billiard in that it deals with a discrete sequence of moves
outside the shape rather than inside of it.