The boundary around a black hole beyond which events cannot affect an outside observer is called the event horizon. It marks the point of no return: after crossing it, no signal, particle, or light ray can travel back out to the wider universe.
The event horizon is not a solid surface or shell. It is a boundary in spacetime created by extreme gravity. For a nonrotating black hole, its radius is called the Schwarzschild radius and depends directly on the object's mass. A black hole with the Sun's mass would have an event-horizon radius of roughly 3 kilometers.
A common misunderstanding is that black holes actively vacuum up everything nearby. In reality, outside the event horizon, their gravity behaves like that of any other object with the same mass. Another misconception concerns time: a distant observer sees falling matter appear to slow, redden, and fade near the horizon, while the falling traveler crosses it after a finite amount of personal time.
The term event horizon applies in broader relativity contexts too, including cosmology, but the black-hole version is the best-known example. The concept grew from early ideas about objects too massive for light to escape and became a precise feature of general relativity.