The event horizon marks the boundary around a black hole beyond which nothing can escape.

Black holes have gravity so strong that once anything crosses a certain boundary, escape becomes impossible. This boundary is called the event horizon. It acts like a one-way door: things can fall in, but nothing can come back out, not even light. This makes the black hole appear completely black, hence the name. The event horizon depends on the black hole’s mass and size; bigger black holes have bigger event horizons.