An event horizon is a boundary in spacetime for a given observer beyond which no electromagnetic energy, including light, can reach the observer.

Light emitted from inside an event horizon will never reach a stationary observer outside the horizon, hence the name black hole. Note the dependency on the observer of the concept of event horizon. For example, an observer, free falling toward a black hole, does not experience an event horizon (e.g. see catastrophic gravitational collapse).

The event horizon for an outside observer really acts as a horizon. He sees an object falling toward the horizon approaching it, but (in his own proper time) never reaching it. In his observations the object goes slower and slower toward the horizon and at the same time the redshift increases beyond bounds to infinity. Also the intensity of the falling object quickly becomes zero. In a finite time the outside observer will receive the last photon from the falling object. He will never see the falling object passing through the event horizon. The event horizon gets its name because information from events that occur within can never reach the outside observer.