Apparent horizon

In general relativity, an apparent horizon is a surface that is the boundary between light rays that are directed outwards and moving outwards, and those directed outward but moving inward.

Apparent horizons are not invariant properties of a spacetime. They are observer-dependent, and in particular they are distinct from event horizons. Within an apparent horizon, light is not moving away from the black hole, whereas in an event horizon, light cannot escape from the black hole. It is possible for light to be currently moving away from the black hole (and so outside the apparent horizon), but in the future will not be able to escape (e.g. because the mass of the black hole is growing) and therefore inside the event horizon. Thus the apparent horizon can be thought of as the boundary of black hole for light at this instant, whereas the event horizon is the boundary of the black hole for light in the future.

See, however, the articles on ergosphere, Cauchy horizon, the Reissner–Nordström solution, photon sphere, Killing horizon and naked singularity; the notion of a horizon in general relativity is subtle, and depends on fine distinctions.