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.
The event horizon is like a one-way membrane in spacetime - you can fall through it, but information about what happens inside can never get back out to tell the tale! Paradoxically, due to time dilation, outside observers would see you frozen at the horizon forever, while you experience falling through normally.
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