Absolute horizon

General relativity
${\displaystyle G_{\mu \nu }+\Lambda g_{\mu \nu }={\kappa }T_{\mu \nu }}$
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In general relativity, an absolute horizon is a boundary in spacetime, defined with respect to the external universe, inside which events cannot affect an external observer. Light emitted inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again by the observer. An absolute horizon is thought of as the boundary of a black hole.

In the context of black holes, the absolute horizon is generally referred to as an event horizon, though this is often used as a more general term for all types of horizons. The absolute horizon is just one type of horizon. For example, important distinctions^[which?] must be made between absolute horizons and apparent horizons; the notion of a horizon in general relativity is subtle, and depends on fine distinctions.