  As demonstrated by Croke and Kleiner, the visual boundary of a CAT(0) group
is not well-defined since quasi-isometric CAT(0) spaces can have
non-homeomorphic boundaries. We introduce a new type of boundary for a CAT(0)
space, called the contracting boundary, made up rays satisfying one of five
hyperbolic-like properties. We prove that these properties are all equivalent
and that the contracting boundary is a quasi-isometry invariant. We use this
invariant to distinguish the quasi-isometry classes of certain right-angled
Coxeter groups.
