What does the term "event horizon" refer to?

The term "event horizon" specifically refers to the boundary surrounding a black hole, marking the point of no return for any object that crosses it. Once an object crosses the event horizon, it cannot escape the gravitational pull of the black hole. This phenomenon arises from the incredibly strong gravitational field produced by the black hole, where the escape velocity exceeds the speed of light. Consequently, all information and matter that cross this boundary are effectively lost to the outside universe.

This concept is crucial in understanding black hole physics and the nature of spacetime within this extreme environment. The event horizon plays a vital role in the calculations and theories surrounding black holes, including their formation and the effects they have on nearby objects and light. It is important to differentiate this definition from other astronomical features. For example, the outer boundary of a star's atmosphere and the edge of a planetary nebula refer to distinct structures that do not involve the unique escape mechanics of black holes. The radius of a neutron star is also a separate concept related to compact stellar remnants but not to the characteristics of black holes or event horizons.