apparent horizon

An apparent horizon is a surface defined in general relativity as the boundary between light rays which are directed outwards and moving outwards, and those which are directed outwards but moving inwards.Apparent horizons are not invariant properties of a spacetime. They are observer-dependent, and in particular they are distinct from absolute horizons.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.
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Noun
1. the line at which the sky and Earth appear to meet
(synonym) horizon, visible horizon, sensible horizon, skyline
(hypernym) line
(part-holonym) perspective, linear perspective

The visible line of demarcation between land/sea and sky. 1/2/73

Généralement, ligne de séparation visible entre d'une part la terre ou la mer et d'autre part le ciel. 1/2/73

ORIZZONTE VISIBILE. ORIZZONTE SENSIBILE. ORIZZONTE APPARENTE