event horizon
Astronomical term for the boundary around a black hole beyond which events cannot affect an outside observer.
See also: 事象の地平線
Once you cross the event horizon of a black hole, not even light can escape.
Synonymous variant; 地平線 is more common for 'horizon' in everyday contexts, but 事象の地平面 is the standard term in physics.
Compound of 事象 (event, phenomenon) + の (possessive particle) + 地平面 (horizontal plane, horizon). A calque of English 'event horizon', used in astrophysics.