The set of equivalence classes of geodesic rays in a Gromov hyperbolic metric space, where two rays are equivalent if they remain a bounded distance apart; it encodes the asymptotic directions and coarse geometry at infinity of the space.

Principle

Identify points at infinity by grouping geodesic rays that fellow-travel up to bounded error; the boundary is a quasi-isometry invariant capturing large-scale, not local, structure.

Demonstration

In the hyperbolic plane H2, geodesic rays correspond to ideal boundary points and the Gromov boundary is homeomorphic to a circle; for a Cayley graph of a finitely generated free group, the Gromov boundary is a Cantor set.

Misapplication

Treating the Gromov boundary as the same as the metric completion boundary or the topological boundary of an embedding; or computing it from small-scale metrics sensitive to local curvature rather than large-scale quasi-geodesic behavior.

Consequence

Correct identification yields a compact topological boundary with an action of isometries-induced dynamics; it informs rigidity, quasi-conformal structure at infinity, and classification of quasi-isometries.

Reversal

Instead of grouping rays that stay close, one could separate rays by their divergent behavior to obtain an interior-type classification; reversing yields a notion sensitive to short-range geometry and loses quasi-isometry invariance.

Boundary

Applies only to Gromov hyperbolic (coarse negative curvature) spaces and to quasi-geodesic frameworks; excludes nonhyperbolic spaces (Euclidean spaces, higher-rank symmetric spaces) where the construction is trivial or inadequate.

Semantic Tension

Tension exists with the visual boundary defined via a specific metric or basepoint and with the limit set of an isometry group; these can coincide in many settings but differ in sensitivity to chosen metrics and coarse structures.

Synthesis

The Gromov boundary is the coarse, quasi-isometry invariant compactification of a hyperbolic space obtained by declaring geodesic rays that fellow-travel to represent the same ideal point, thereby encoding the space's asymptotic geometry.