The event horizon is the boundary around a black hole beyond which light cannot escape.
An event horizon is not a solid surface. It is a boundary in spacetime defined by the black hole’s gravity and the structure of possible future paths. Once an object crosses it, every future-directed path leads deeper inward, so an outside observer cannot receive a signal from that object.
The idea follows from general relativity, Albert Einstein’s 1915 theory of gravity. For a nonrotating black hole, the event-horizon radius is called the Schwarzschild radius and depends on the black hole’s mass. A more massive black hole has a larger event horizon.
The event horizon should not be confused with the singularity, which is the predicted central region where classical descriptions break down, or with an accretion disk, which is hot matter orbiting outside the boundary. Black holes are detected through effects such as stellar orbits, gravitational waves, and radiation from nearby matter rather than by seeing the horizon directly.