The boundary surrounding a black hole beyond which nothing can escape, not even light. It represents the point of no return where the escape velocity exceeds the speed of light.
The term was coined by Wolfgang Rindler in the 1950s, combining 'event' (referring to anything that happens) with 'horizon' (a boundary beyond which you cannot see). It elegantly captures the idea of a boundary in spacetime beyond which events cannot influence the outside universe.
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