event horizon

In general relativity, an event horizon is a boundary in spacetime, an area surrounding a black hole, inside which events cannot affect an outside observer. Light emitted from inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side appears to freeze in place, with its image becoming more redshifted as time proceeds.

More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Still other distinct notions include the Cauchy and Killing horizon; the photon spheres and ergospheres of the Reissner-Nordström solution; particle and cosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.