Event Horizon

The boundary in spacetime separating events that can influence distant observers from those that cannot; specifically, the surface generated by null geodesics that fail to reach future null infinity, commonly surrounding a black hole and marking the one-way causal surface.

The global null hypersurface that forms the boundary of the region from which no causal (timelike or null) signal can reach distant observers; operationally, the event horizon is the boundary of the causal past of future null infinity in a spacetime and is therefore a teleological, global feature of the spacetime geometry.